add support for tensor learning rate (vs scalar) (#7633)
This change is intended to help enable support for using a tensor
learning rate value vs a scalar ones.
We found this helpful in cases where the optimizer is torch.compiled (in
such cases changing the scalar LR value could cause recompilation
degrading the performance).
The implementation allows the model script to determine the type of LR
value used by setting the initial value.
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
