First draft of the docs for TF + Datasets

docs/source/_toctree.yml

@@ -33,6 +33,8 @@
     title: Process image data
   - local: stream
     title: Stream
+  - local: use_with_tensorflow
+    title: Use with TensorFlow
   - local: share
     title: Share
   - local: dataset_script

docs/source/use_with_tensorflow.mdx

@@ -0,0 +1,196 @@
+# Using Datasets with TensorFlow
+
+This document is a quick introduction to using `datasets` with TensorFlow, with a particular focus on how to get
+`tf.Tensor` objects out of our datasets, and how to stream data from Hugging Face `Dataset` objects to Keras methods
+like `model.fit()`.
+
+### Dataset format
+
+By default, datasets return regular Python objects: integers, floats, strings, lists, etc.
+
+To get TensorFlow tensors instead, you can set the format of the dataset to `tf`:
+
+```py
+>>> from datasets import Dataset
+>>> data = [[1, 2],[3, 4]]
+>>> ds = Dataset.from_dict({"data": [[1, 2],[3, 4]]})
+>>> ds = ds.with_format("tf")
+>>> ds[0]
+{'data': <tf.Tensor: shape=(2,), dtype=int64, numpy=array([1, 2])>}
+>>> ds[:2]
+{'data': <tf.Tensor: shape=(2, 2), dtype=int64, numpy=
+array([[1, 2],
+       [3, 4]])>}
+```
+
+<Tip>
+
+A [`Dataset`] object is a wrapper of an Arrow table, which allows fast reads from arrays in the dataset to TensorFlow tensors.
+
+</Tip>
+
+This can be useful for converting your dataset to a dict of `Tensor` objects, or for writing a generator to load TF
+samples from it. If you wish to convert the entire dataset to `Tensor`, simply query the full dataset:
+
+```py
+>>> ds[:]
+{'data': <tf.Tensor: shape=(2, 2), dtype=int64, numpy=
+array([[1, 2],
+       [3, 4]])>}
+```
+
+### N-dimensional arrays
+
+If your dataset consists of N-dimensional arrays, you will see that by default they are considered as nested lists.
+In particular, a TensorFlow formatted dataset outputs a `RaggedTensor` instead of a single tensor:
+
+```py
+>>> from datasets import Dataset
+>>> data = [[[1, 2],[3, 4]],[[5, 6],[7, 8]]]
+>>> ds = Dataset.from_dict({"data": data})
+>>> ds = ds.with_format("tf")
+>>> ds[0]
+{'data': <tf.RaggedTensor [[1, 2], [3, 4]]>}
+```
+
+To get a single tensor, you must explicitly use the Array feature type and specify the shape of your tensors:
+
+```py
+>>> from datasets import Dataset, Features, Array2D
+>>> data = [[[1, 2],[3, 4]],[[5, 6],[7, 8]]]
+>>> features = Features({"data": Array2D(shape=(2, 2), dtype='int32')})
+>>> ds = Dataset.from_dict({"data": data}, features=features)
+>>> ds = ds.with_format("tf")
+>>> ds[0]
+{'data': <tf.Tensor: shape=(2, 2), dtype=int64, numpy=
+ array([[1, 2],
+        [3, 4]])>}
+>>> ds[:2]
+{'data': <tf.Tensor: shape=(2, 2, 2), dtype=int64, numpy=
+ array([[[1, 2],
+         [3, 4]],
+
+        [[5, 6],
+         [7, 8]]])>}
+```
+
+
+### Other feature types
+
+[`ClassLabel`] data are properly converted to tensors:
+
+```py
+>>> from datasets import Dataset, Features, ClassLabel
+>>> data = [0, 0, 1]
+>>> features = Features({"data": ClassLabel(names=["negative", "positive"])})
+>>> ds = Dataset.from_dict({"data": data}, features=features) 
+>>> ds = ds.with_format("tf")  
+>>> ds[:3]
+{'data': <tf.Tensor: shape=(3,), dtype=int64, numpy=array([0, 0, 1])>
+```
+
+Strings are also supported:
+
+```py
+>>> from datasets import Dataset, Features 
+>>> text = ["foo", "bar"]
+>>> data = [0, 1] 
+>>> ds = Dataset.from_dict({"text": text, "data": data})  
+>>> ds = ds.with_format("tf") 
+>>> ds[:2]
+{'text': <tf.Tensor: shape=(2,), dtype=string, numpy=array([b'foo', b'bar'], dtype=object)>,
+ 'data': <tf.Tensor: shape=(2,), dtype=int64, numpy=array([0, 1])>}
+```
+
+You can also explicitly format certain columns and leave the other columns unformatted:
+
+```py
+>>> ds = ds.with_format("tf", columns=["data"], output_all_columns=True)
+>>> ds[:2]
+{'data': <tf.Tensor: shape=(2,), dtype=int64, numpy=array([0, 1])>,
+ 'text': ['foo', 'bar']}
+```
+
+The [`Image`] and [`Audio`] feature types are not supported yet.
+
+## Data loading
+
+Although you can load individual samples and batches just by indexing into your dataset, this won't work if you want
+to use Keras methods like `fit()` and `predict()`. You could write a generator function that shuffles and loads batches
+from your dataset and `fit()` on that, but that sounds like a lot of unnecessary work. Instead, if you want to stream
+data from your dataset on-the-fly, we recommend converting your dataset to a `tf.data.Dataset` using the
+`to_tf_dataset()` method.
+
+The `tf.data.Dataset` class covers a wide range of use-cases - it is often created from Tensors in memory, or using a load function to read files on disc
+or external storage. The dataset can be transformed arbitrarily with the `map()` method, or methods like `batch()`
+and `shuffle()` can be used to create a dataset that's ready for training. These methods do not modify the stored data
+in any way - instead, the methods build a data pipeline graph that will be executed when the dataset is iterated over,
+usually during model training or inference. This is different from the `map()` method of Hugging Face `Dataset` objects,
+which runs the map function immediately and saves the new or changed columns.
+
+Since the entire data preprocessing pipeline can be compiled in a `tf.data.Dataset`, this approach allows for massively
+parallel, asynchronous data loading and training. However, the requirement for graph compilation can be a limitation,
+particularly for Hugging Face tokenizers, which are usually not (yet!) compilable as part of a TF graph. As a result, 
+we usually advise pre-processing the dataset as a Hugging Face dataset, where arbitrary Python functions can be
+used, and then converting to `tf.data.Dataset` afterwards using `to_tf_dataset()` to get a batched dataset ready for
+training. To see examples of this approach, please see the [examples](https://github.com/huggingface/transformers/tree/main/examples) or [notebooks](https://huggingface.co/docs/transformers/notebooks) for `transformers`.
+
+### Using `to_tf_dataset()`
+
+Using `to_tf_dataset()` is straightforward. Once your dataset is preprocessed and ready, simply call it like so:
+
+```py
+>>> from datasets import Dataset
+>>> data = {"inputs": [[1, 2],[3, 4]], "labels": [0, 1]}
+>>> ds = Dataset.from_dict(data)
+>>> tf_ds = ds.to_tf_dataset(
+            columns=["inputs"],
+            label_cols=["labels"],
+            batch_size=2,
+            shuffle=True
+            )
+```
+
+The returned `tf_ds` object here is now fully ready to train on, and can be passed directly to `model.fit()`! Note
+that you set the batch size when creating the dataset, and so you don't need to specify it when calling `fit()`:
+
+```py
+>>> model.fit(tf_ds, epochs=2)
+```
+
+For a full description of the arguments, please see the [`~Dataset.to_tf_dataset`] documentation. In many cases,
+you will also need to add a `collate_fn` to your call. This is a function that takes multiple elements of the dataset
+and combines them into a single batch. When all elements have the same length, the built-in default collator will
+suffice, but for more complex tasks a custom collator may be necessary. In particular, many tasks have samples
+with varying sequence lengths which will require a [data collator](https://huggingface.co/docs/transformers/main/en/main_classes/data_collator) that can pad batches correctly. You can see examples
+of this in the `transformers` NLP [examples](https://github.com/huggingface/transformers/tree/main/examples) and
+[notebooks](https://huggingface.co/docs/transformers/notebooks), where variable sequence lengths are very common.
+
+### When to use to_tf_dataset
+
+The astute reader may have noticed at this point that we have offered two approaches to achieve the same goal - if you
+want to pass your dataset to a TensorFlow model, you can either convert the dataset to a `Tensor` or `dict` of `Tensors`
+using `.with_format('tf')`, or you can convert the dataset to a `tf.data.Dataset` with `to_tf_dataset()`. Either of these
+can be passed to `model.fit()`, so which should you choose?
+
+The key thing to recognize is that when you convert the whole dataset to `Tensor`s, it is static and fully loaded into
+RAM. This is simple and convenient, but if any of the following apply, you should probably use `to_tf_dataset()`
+instead:
+
+- Your dataset is too large to fit in RAM. `to_tf_dataset()` streams only one batch at a time, so even very large
+  datasets can be handled with this method.
+- You want to apply random transformations using `dataset.with_transform()` or the `collate_fn`. This is
+  common in several modalities, such as image augmentations when training vision models, or random masking when training
+  masked language models. Using `to_tf_dataset()` will apply those transformations
+  at the moment when a batch is loaded, which means the same samples will get different augmentations each time
+  they are loaded. This is usually what you want.
+- Your data has a variable dimension, such as input texts in NLP that consist of varying
+  numbers of tokens. When you create a batch with samples with a variable dimension, the standard solution is to
+  pad the shorter samples to the length of the longest one. When you stream samples from a dataset with `to_tf_dataset`,
+  you can apply this padding to each batch via your `collate_fn`. However, if you want to convert
+  such a dataset to dense `Tensor`s, then you will have to pad samples to the length of the longest sample in *the
+  entire dataset!* This can result in huge amounts of padding, which wastes memory and reduces your model's speed.
+
+### Caveats and limitations
+
+Right now, `to_tf_dataset()` always return a batched dataset - we will add support for unbatched datasets soon!