Enable T5 and similar models

[guangy10]

This PR enables T5 and similar models e2e to ExecuTorch.

• This PR relies on Export T5 (encoder-decoder) to ExecuTorch transformers#36486 to export the T5 model
• This PR will fail on CI because the above PR is not available in transformers
• This PR is tested with both executorch and transformers installed from main

Setup:

executorch            0.6.0a0+84273f4
torch                 2.7.0.dev20250131
transformers          4.50.0.dev0        /Users/guangyang/transformers

Test:
RUN_SLOW=1 pytest tests/models/test_modeling_t5.py -s -v

============================================================================================= test session starts =============================================================================================
platform darwin -- Python 3.11.11, pytest-8.3.4, pluggy-1.5.0 -- /Users/guangyang/miniconda3/envs/executorch/bin/python
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase(PosixPath('/Users/guangyang/optimum-executorch/.hypothesis/examples'))
rootdir: /Users/guangyang/optimum-executorch
configfile: pyproject.toml
plugins: rerunfailures-15.0, hypothesis-6.126.0, xdist-3.6.1
collecting ... 
collected 2 items

tests/models/test_modeling_t5.py::ExecuTorchModelIntegrationTest::test_t5_export import
/Users/guangyang/miniconda3/envs/executorch/lib/python3.11/site-packages/executorch/exir/emit/_emitter.py:1592: UserWarning: Mutation on a buffer in the model is detected. ExecuTorch assumes buffers that are mutated in the graph have a meaningless initial state, only the shape and dtype will be serialized, unless a pass which sets meta["et_init_buffer"] to True such as InitializedMutableBufferPass is run.
  warnings.warn(
PASSED
tests/models/test_modeling_t5.py::ExecuTorchModelIntegrationTest::test_t5_summarization No ExecuTorch files were found for google-t5/t5-small, setting `export=True` to convert the model to the ExecuTorch IR.
[program.cpp:136] InternalConsistency verification requested but not available
[program.cpp:136] InternalConsistency verification requested but not available
PASSED

============================================================================================== warnings summary ===============================================================================================
================================================================================= 2 passed, 227 warnings in 142.02s (0:02:22) =================================================================================

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[guangy10]

Squeeze more enabled models (encoder-only, decoder-only, encoder-decoder, etc.) to this PR for CI testing only. I will publish a new PR with the updated doc/instructions shortly.

[guangy10]

Moving to #36