Fix WarmupCosineLR multi-group initialization (#7969)
`WarmupCosineLR` returned a singleton pre-start LR list even when the
optimizer had multiple parameter groups. Because scheduler
initialization applies LRs with `zip(param_groups, lrs)`, only group 0
was updated and later groups kept their base LR before the first
optimizer step.
The fix changes the pre-start scheduler outputs to match the multi-group
contract by returning scalar `0.0` from `get_lr_ratio()` and a
zero-filled LR list sized to `self.org_lrs`.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
