[BE][FSDP] Fix that MP config not being passed to FSDP (#80869)
The mixed precision config was being passed to the wrapped module instead of propagating to the FSDP constructor. This fix will become obsolete with the `FSDPTestModel` PR higher in the stack, but it is still worth fixing by itself.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80869
Approved by: https://github.com/rohan-varma
