Adding doctest for fill-mask pipeline.

src/transformers/pipelines/fill_mask.py

@@ -36,6 +36,22 @@ class FillMaskPipeline(Pipeline):
     Masked language modeling prediction pipeline using any `ModelWithLMHead`. See the [masked language modeling
     examples](../task_summary#masked-language-modeling) for more information.
 
+    Example:
+
+    ```python
+    >>> from transformers import pipeline
+
+    >>> fill_masker = pipeline(model="bert-base-uncased")
+    >>> potential_words = fill_masker("This is a simple [MASK].")
+
+    >>> from transformers.testing_utils import nested_simplify
+
+    >>> nested_simplify(potential_words)  # The scores might vary slightly across pytorch/tensorflow versions.
+    [{'score': 0.042, 'token': 3291, 'token_str': 'problem', 'sequence': 'this is a simple problem.'}, {'score': 0.031, 'token': 3160, 'token_str': 'question', 'sequence': 'this is a simple question.'}, {'score': 0.03, 'token': 8522, 'token_str': 'equation', 'sequence': 'this is a simple equation.'}, {'score': 0.027, 'token': 2028, 'token_str': 'one', 'sequence': 'this is a simple one.'}, {'score': 0.024, 'token': 3627, 'token_str': 'rule', 'sequence': 'this is a simple rule.'}]
+    ```
+
+    [Using pipelines in a webserver or with a dataset](../pipeline_tutorial)
+
     This mask filling pipeline can currently be loaded from [`pipeline`] using the following task identifier:
     `"fill-mask"`.