Set should_evaluate=eval_on_start on train start
Author: I-l-l-I
Pull request: yes

# What does this PR do?
Set `control.should_evaluate=True` when `eval_on_start=True` on train start.
### Motivation
Currently if `eval_on_start=True`, `control.should_evaluate` stays false during the first evaluation, which doesn't make sense.
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## Who can review?

@zach-huggingface and @SunMarc 

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Comments:
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@SunMarc Thank you for your reply. So `should_evaluate` just means that the model needs to be evaluated after a training step. If I need to determine if the model is in evaluation mode, I need to use `model.training` instead of `should_evaluate`, right?

> So should_evaluate just means that the model needs to be evaluated after a training step. If I need to determine if the model is in evaluation mode, I need to use model.training instead of should_evaluate, right?

Yeah that's right.
Using model.training should work pretty well indeed. It is not recommend to use should_evaluate for that 

Thanks for clarifying!