Load sharded pt model to flax (from the hub )
# What does this PR do?

Implements the automatic conversion from `pt` to `flax` if the model is sharded but does not already have flax weights. This could be useful for automatic conversion 

## Who can review?

A few tests still need to be implemented, for now works well with `hf-internal-testing/tiny-random-bert-sharded`


_The documentation is not available anymore as the PR was closed or merged._<|||||>Follows  #17537