Allow smaller `t_max` in schedulers

[priba]

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Problem

The PR #3115 has added some checks to the scheduler. More precisely, the _raise_if_max_duration_exceeds_t_max raise the error:

't_max {t_max} must be greater than or equal to max_duration {max_dur}. Otherwise, the LR schedule will  not be defined for the entire training duration.'

when the duration of the scheduler is less than the max duration of the trainer.

Expected behavior

I don't think this is an error but maybe a warning. Lets say we want to use a liner scheduler only for the first half of the training, I should be able to setup t_max to the expected length which is less than max_dur isn't it?

Additional context

Tagging the reviewer and author of the PR for visibility and get their insights @b-chu @snarayan21