Remove NumPy from Callback scripts

[ishandutta0098]

What does this PR do?

This PR is a part of #16649

I have replaced the use of np.Inf and np.nan with float('inf') and float('nan') for the following scripts:

• model_checkpoint.py
• early_stopping.py
• test_early_stopping.py

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Requesting Review: @carmocca @Borda