cross platform from_pretrained
# What does this PR do?
Allows loading sharded checkpoints in TF models. Should fix #19965
- [x] `from_pt=True`
- [ ] `from_flax=True`

cc @sgugger just FYI 
Works great for sharded `pytorch` since a utility was already implemented. Though we are not gonna push for `Flax`, would still help to have the support already! 
```python 
from transformers import TFT5ForConditionalGeneration
MODEL_NAME = "google/flan-t5-xl"
m = TFT5ForConditionalGeneration.from_pretrained(MODEL_NAME, from_pt=True)
```
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Just need to remove the `# TODOs` 
