[MPS] Move mps_linear to mps dispatch key (#80068)
Fixes #77394
This is related to #79920 which adds linear support for nested tensors. Codegen still throws an assert stoping this from compiling. However I tested locally by commenting out this assert: https://github.com/pytorch/pytorch/blob/61305cd638b6fcd73a0b66b4cde7014fecb9e8ce/tools/autograd/gen_variable_type.py#L798
and the intended behavior appears to be working. I am not sure what changes need to be made to codegen to make this work.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80068
Approved by: https://github.com/albanD, https://github.com/malfet, https://github.com/kulinseth
