fix: cosine annealing linear warmup lr

[clumsy]

What does this PR do ?

CosineAnnealing LR with linear warmup and constant_steps is 0 on the first step. This causes issues for AdamW optimizer which uses LR for step_size in the denominator. This is due to the fact on first step when this method is called step=0 before the first optimizer step is complete.

Using the same protection as in:

• NeMo/nemo/core/optim/lr_scheduler.py

  Line 81 in d2f3f49

  lr_val = (step + 1) / (self.warmup_steps + 1)

• NeMo/nemo/core/optim/lr_scheduler.py

  Line 315 in d2f3f49

  lr_val = (step + 1) / (self.warmup_steps + 1)

• NeMo/nemo/core/optim/lr_scheduler.py

  Line 611 in d2f3f49

  denom = ((step + 1) / (self.warmup_steps + 1)) ** 0.5

Updated unit tests not to tolerate LR=0.

Collection: [Note which collection this PR will affect]

Core

Changelog

• fix cosine annealing linear warmup LR

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