Change state dict loading default to strict

[dakinggg]

What does this PR do?

This PR changes the trainer default for load_strict_model_weights to True. We would previously (and will still) emit a warning if there are unexpected or missing keys in the checkpoint, just now we will default to erroring out. This matches torch's default (https://pytorch.org/docs/stable/generated/torch.nn.Module.html#torch.nn.Module.load_state_dict), and helps prevent silent failures to load the model correctly.

TODO:

• update notebooks

What issue(s) does this change relate to?

Closes CO-2067

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[eracah]

LGTM