[MPS] Fixes for LSTM. (#94889)
- Backward pass has to give explicit bias tensor of zeros if none is passed to the op or the bias gradient will not be calculated.
- Fixed bias tensor mistakenly getting overwritten to zeros
- Fixes crash when lstm op called with has_biases set to false. Change takes into account the changed shape of the input params TensorList depending on the bias flag.
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94889
Approved by: https://github.com/DenisVieriu97
