To change WarmUp Scheduler with ConstantLR and LinearLR (#64395)
Summary:
Partially unblocks https://github.com/pytorch/vision/issues/4281
Previously we have added WarmUp Schedulers to PyTorch Core in the PR : https://github.com/pytorch/pytorch/pull/60836 which had two mode of execution - linear and constant depending on warming up function.
In this PR we are changing this interface to more direct form, as separating linear and constant modes to separate Schedulers. In particular
```Python
scheduler1 = WarmUpLR(optimizer, warmup_factor=0.1, warmup_iters=5, warmup_method="constant")
scheduler2 = WarmUpLR(optimizer, warmup_factor=0.1, warmup_iters=5, warmup_method="linear")
```
will look like
```Python
scheduler1 = ConstantLR(optimizer, warmup_factor=0.1, warmup_iters=5)
scheduler2 = LinearLR(optimizer, warmup_factor=0.1, warmup_iters=5)
```
correspondingly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64395
Reviewed By: datumbox
Differential Revision: D30753688
Pulled By: iramazanli
fbshipit-source-id: e47f86d12033f80982ddf1faf5b46873adb4f324
