Cherry pick `Update T5 DataModule regarding Pretrain/Finetune validate (11584)` into `r2.1.0`
<!-- pylint-output-strict-mode-false -->

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

---

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

```
************* Module nemo.collections.llm.t5.data.mock
nemo/collections/llm/t5/data/mock.py:33:0: C0115: Missing class docstring (missing-class-docstring)
nemo/collections/llm/t5/data/mock.py:73:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/mock.py:84:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/mock.py:89:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/mock.py:94:4: C0116: Missing function or method docstring (missing-function-docstring)
************* Module nemo.collections.llm.t5.data.pre_training
nemo/collections/llm/t5/data/pre_training.py:302:0: C0301: Line too long (124/119) (line-too-long)
nemo/collections/llm/t5/data/pre_training.py:309:0: C0301: Line too long (133/119) (line-too-long)
nemo/collections/llm/t5/data/pre_training.py:166:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/pre_training.py:223:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/pre_training.py:226:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/pre_training.py:229:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/pre_training.py:246:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/collections/llm/t5/data/pre_training.py:299:4: C0116: Missing function or method docstring (missing-function-docstring)

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Your code has been rated at 9.37/10
```

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Thank you for improving NeMo's documentation! Already merged cherrypick in different PR
beep boop [🤖]: Hi @huvunvidia 👋,
    
    we've cherry picked #11584 into  for you! 🚀
    
    Please review and approve this cherry pick by your convenience!