fix typing bug of LambdaLR.__init__ (#33271)
Summary:
## problem
```python
class LambdaLR(_LRScheduler):
"""Sets the learning rate of each parameter group to the initial lr
times a given function. When last_epoch=-1, sets initial lr as lr.
Args:
optimizer (Optimizer): Wrapped optimizer.
lr_lambda (function or list): A function which computes a multiplicative
factor given an integer parameter epoch, or a list of such
functions, one for each group in optimizer.param_groups.
last_epoch (int): The index of last epoch. Default: -1.
Example:
>>> # Assuming optimizer has two groups.
>>> lambda1 = lambda epoch: epoch // 30
>>> lambda2 = lambda epoch: 0.95 ** epoch
>>> scheduler = LambdaLR(optimizer, lr_lambda=[lambda1, lambda2])
>>> for epoch in range(100):
>>> train(...)
>>> validate(...)
>>> scheduler.step()
"""
```
`LambdaLR` takes a lambda that returns a float and takes a int, or a list of such lambdas.
## related issue
Resolve https://github.com/pytorch/pytorch/issues/32645
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33271
Differential Revision: D19878665
Pulled By: vincentqb
fbshipit-source-id: 50b16caea13de5a3cbd187e688369f33500499d0
