Utils mixin for `Module` (sub)classes (num params etc.)

[lkct]

Ref https://github.com/april-tools/cirkit/pull/110#discussion_r1259570153.

We can add some utils for some commonly used properties/methods on layers and models (which subclass Module).

• num_params for the number of trainable params in the module (and a flag to exclude the params set to requires_grad=False)
• whatever else we think useful

One option is to use util functions as in the discussion ref above. Another option is to use mixin classes (mypy's guide on how to type them: https://mypy.readthedocs.io/en/stable/more_types.html#mixin-classes)

[lkct]

cc @loreloc Continuing the discussion in the previous PR, I have another option of mixin classes. Maybe it's better? (in the sense that we can add multiple utils with one mixin)

[loreloc]

I think it's better to have simple utility functions for now. It seems we can always switch to mixin classes easily, if we notice we need many utility functions like num_params.