To add SequentialLR to PyTorch Core Schedulers (#64037)

To add SequentialLR to PyTorch Core Schedulers (#64037)

Summary:
Partially resolves https://github.com/pytorch/vision/issues/4281

In this PR we are proposing a new scheduler --SequentialLR-- which enables list of different schedulers called in different periods of the training process.

The main motivation of this scheduler is recently gained popularity of warming up phase in the training time. It has been shown that having a small steps in initial stages of training can help convergence procedure get faster.

With the help of SequentialLR we mainly enable to call a small constant (or linearly increasing) learning rate followed by actual target learning rate scheduler.

```PyThon
scheduler1 = ConstantLR(optimizer, factor=0.1, total_iters=2)
scheduler2 = ExponentialLR(optimizer, gamma=0.9)
scheduler = SequentialLR(optimizer, schedulers=[scheduler1, scheduler2], milestones=[5])

for epoch in range(100):
    train(...)
    validate(...)
    scheduler.step()
```

which this code snippet will call `ConstantLR` in the first 5 epochs and will follow up with `ExponentialLR` in the following epochs.

This scheduler could be used to provide call of any group of schedulers next to each other. The main consideration we should make is every time we switch to a new scheduler we assume that new scheduler starts from the beginning- zeroth epoch.

We also add Chained Scheduler to `optim.rst` and `lr_scheduler.pyi` files here.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/64037

Reviewed By: albanD

Differential Revision: D30841099

Pulled By: iramazanli

fbshipit-source-id: 94f7d352066ee108eef8cda5f0dcb07f4d371751