[FastSpeech2] Variance can be None but it is used in `forward()`

[SamuelLarkin]

FastSpeech2's variance adaptor can be set to None and later we try to use it in the forward pass.

1. When constructing a FastSpeech2(), on line https://github.com/roedoejet/FastSpeech2_lightning/blob/main/fs2/model.py#L33 we expect a stats and by default it is None.
2. We keep a reference to stats https://github.com/roedoejet/FastSpeech2_lightning/blob/main/fs2/model.py#L49.
3. If stats is None on line https://github.com/roedoejet/FastSpeech2_lightning/blob/main/fs2/model.py#L79, we set the variance_adaptor to None at line https://github.com/roedoejet/FastSpeech2_lightning/blob/main/fs2/model.py#L84. We then log an error.
4. During the forward() pass, we calculate variance at line https://github.com/roedoejet/FastSpeech2_lightning/blob/main/fs2/model.py#L162 regardless if the member is None

What should happen when the variance is not set?