Follow-up's to #681

[ascillitoe]

Tracker for three items identified in #681.

Item 1 (#681 (comment))

Shallow copies are currently used to set and handle config, for example:

 cfg = self.config.copy()

This is OK if all any dictionary items are replaced, but might lead to unexpected behaviour if any items are mutated. We should audit all instances and replace with deepcopy where necessary.

Item 2 (#681 (comment))

We should replace kernel_a: Union[nn.Module, str] = 'rbf''s with Literal's in the kernel submodules, since rbf is the only permitted string value.

Item 3 (#681 (comment))

Possibly refactor if statements, e.g.:

Not a massive deal but rather than:

if backend == Framework.TENSORFLOW:
        kernel = load_kernel_config_tf(cfg)
elif backend == Framework.PYTORCH:
        kernel = load_kernel_config_pt(cfg)
else:  # backend=='keops'
        kernel = load_kernel_config_ke(cfg)

we might be able to do:

return {
    Framework.TENSORFLOW: load_kernel_config_tf,
    Framework.PYTORCH: load_kernel_config_pt,
    Framework.KEOPS: load_kernel_config_ko
}[backend]()