add transformer class for review
added a unit test. This is the necessary test to ensure the forward pass of Transformer class succeed. 

The multiple conditions from difference encoders failed the tests (`test_forward_causal_self_attn_and_has_xattn`). It seems a list of tensors are not supported. @paarthneekhara could you pls verify?

pls run `pytest -s -vvv tests/collections/tts/modules/test_tts_new_transformer.py` locally to test the code. <!-- pylint-output-strict-mode-true -->

beep boop 🤖: 🚨 The following files must be fixed before merge!

---

Your code was analyzed with PyLint. The following annotations have been identified:

```
************* Module nemo.collections.tts.modules.transformer_2412
nemo/collections/tts/modules/transformer_2412.py:26:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:85:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/tts/modules/transformer_2412.py:94:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:138:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:171:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/tts/modules/transformer_2412.py:191:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/tts/modules/transformer_2412.py:195:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/tts/modules/transformer_2412.py:280:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:340:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:398:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:471:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/tts/modules/transformer_2412.py:535:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/tts/modules/transformer_2412.py:626:4: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 9.47/10
```

---

Mitigation guide:

* Add sensible and useful docstrings to functions and methods
* For trivial methods like getter/setters, consider adding `# pylint: disable=C0116` inside the function itself
* To disable multiple functions/methods at once, put a `# pylint: disable=C0116` before the first and a `# pylint: enable=C0116` after the last.

By applying these rules, we reduce the occurance of this message in future.

Thank you for improving NeMo's documentation! @XuesongYang We need to pass `multi_encoder_mapping` to the forward function as well for multi-encoder case. I have updated the test case with some comments (that still need to be incorporated) and fixes. Also corrected the `x = (x + x_) * x_mask.unsqueeze(-1)` bug which I believe was inserted when the mask was flipped. 

FYI, I tested this new transformer code training and inference with t5tts locally, and it seems to be working fine. Also for a fixed set of weights, the transformer implementation in `experimentalt5tts` and this branch, give the same output, so I think we should be good. 
Added the transformer stack currently being used in T5TTS - Identify unused code paths, clean up the code and see what modules can be reused.