Fix Edge Case when iter is at MAX_ITER

[johnanthonyjose]

Context:
I used ConstantParamScheduler as my baseline scheduler in a detectron2-based repository.

For example: cfg.SOLVER.MAX_ITER = 80,000

When self.iter is at 79,999 and triggers after_step(), it will display a runtime error as shown below:

RuntimeError: Invalid where parameter for scheduler: 1.0

I've learned that it's due to the fact that the self.last_epoch of the scheduler is always ahead by 1 relative to self.iter of the trainer.

To resolve this, by reducing where by self.EPSILON_WHERE, the result value becomes 0.999999.

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