Add FastSpeech2Conformer (#23439)

* start - docs, SpeechT5 copy and rename

* add relevant code from FastSpeech2 draft, have tests pass

* make it an actual conformer, demo ex.

* matching inference with original repo, includes debug code

* refactor nn.Sequentials, start more desc. var names

* more renaming

* more renaming

* vocoder scratchwork

* matching vocoder outputs

* hifigan vocoder conversion script

* convert model script, rename some config vars

* replace postnet with speecht5's implementation

* passing common tests, file cleanup

* expand testing, add output hidden states and attention

* tokenizer + passing tokenizer tests

* variety of updates and tests

* g2p_en pckg setup

* import structure edits

* docstrings and cleanup

* repo consistency

* deps

* small cleanup

* forward signature param order

* address comments except for masks and labels

* address comments on attention_mask and labels

* address second round of comments

* remove old unneeded line

* address comments part 1

* address comments pt 2

* rename auto mapping

* fixes for failing tests

* address comments part 3 (bart-like, train loss)

* make style

* pass config where possible

* add forward method + tests to WithHifiGan model

* make style

* address arg passing and generate_speech comments

* address Arthur comments

* address Arthur comments pt2

* lint  changes

* Sanchit comment

* add g2p-en to doctest deps

* move up self.encoder

* onnx compatible tensor method

* fix is symbolic

* fix paper url

* move models to espnet org

* make style

* make fix-copies

* update docstring

* Arthur comments

* update docstring w/ new updates

* add model architecture images

* header size

* md wording update

* make style

docs/source/en/index.md

@@ -132,6 +132,7 @@ Flax), PyTorch, and/or TensorFlow.
 |                           [ESM](model_doc/esm)                           |       ✅        |         ✅         |      ❌      |
 |              [FairSeq Machine-Translation](model_doc/fsmt)               |       ✅        |         ❌         |      ❌      |
 |                        [Falcon](model_doc/falcon)                        |       ✅        |         ❌         |      ❌      |
+|         [FastSpeech2Conformer](model_doc/fastspeech2_conformer)          |       ✅        |         ❌         |      ❌      |
 |                       [FLAN-T5](model_doc/flan-t5)                       |       ✅        |         ✅         |      ✅      |
 |                      [FLAN-UL2](model_doc/flan-ul2)                      |       ✅        |         ✅         |      ✅      |
 |                      [FlauBERT](model_doc/flaubert)                      |       ✅        |         ✅         |      ❌      |