Fix initial learning rate (#453)

Summary:
There was a very subtle bug here 😢When we recently removed this line (7633129b), it meant that the learning rate scheduler didn't get initialized until after the first update. Unfortunately pytorch optimizers store the learning rate in their internal state, so some learning rate schedulers use their `__init__` method to reset the learning rate to some sane initial value. This is especially problematic for LR schedulers that include a warmup, where the Optimizer is likely to contain the peak learning rate at initialization, and it's only in the LR scheduler's `__init__` that the (much smaller) warmup value is set.

For example, the inverse_sqrt scheduler resets the learning rate upon initialization:
https://github.com/pytorch/fairseq/blob/7853818c2e33a63ec17a31bcfe20e4fc75d94130/fairseq/optim/lr_scheduler/inverse_square_root_schedule.py#L48-L50

**Impact:** For the last ~1.5 weeks, the first training update would use the optimizer's default learning rate instead of the initial rate set by the LR scheduler. All subsequent updates used the correct learning rates. This primarily affects LR schedulers with warmups.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/453

Differential Revision: D13704453

Pulled By: myleott

fbshipit-source-id: a946da30100f837c66bdc6b9b77b014ab4eb8764