Add support for `ChainedScheduler`s in `LitModular`.

[dxoigmn]

Right now LitModular does not support ChainedScheduler. This is because a ChainedScheduler does not take an optimizer, but rather a list of schedulers:
https://pytorch.org/docs/stable/generated/torch.optim.lr_scheduler.ChainedScheduler.html

LitModular assumes lr_scheduler is a callable that receives an Optimizer:

MART/mart/models/modular.py

Line 122 in 41ad18e

config["lr_scheduler"] = self.lr_scheduler(config["optimizer"])

My sense is the best way to fix this is to allow lr_scheduler to be a list and we can create the underlying ChainedScheduler within LitModular.