Added Module.__check_init__ for after-initialisation checking of invariants. See #472.

[patrick-kidger]

As discussed in #472, it would be nice to have a way to check invariants after a class has been instantiated. Typical use cases are something like:

class A(eqx.Module):
    x: int

    def __check_init__(self):
        if x < 0:
            raise ValueError("x must be non-negative")

This PR adds __check_init__, which is automatically called after initialisation.

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Now you may wonder, why not use __post_init__, as provided by dataclasses? It turns out that this use-case isn't well-served by __post_init__. The reason is that __post_init__ is not called if a subclass defines an __init__ or __post_init__ method of its own, and forgets to call super().__post_init__(). Any checks placed in __post_init__ are silently never run!

Indeed, it turns out exactly this issue has existed in Diffrax for some time: patrick-kidger/diffrax#308.

The advantage of __check_init__ is that all instances across the whole MRO are automatically ran. That is, all superclass __check_init__ are automatically ran, without needing to call super(). This makes it impossible for a downstream class to silently avoid checking the invariants of its parent class.

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Note that __check_init__ is ran after the class is completely initialised. That is, it is no longer possible to assign self.foo = bar inside of __check_init__.

This is a deliberate constraint, to prevent parent classes from silently performing any behaviour that may be surprising for its child class. This is particularly important, as a child class has no way to override the __check_init__ of its parent.

As the name suggests, this method is intended for checking invariants, not arbitrary postprocessing. (Use __post_init__ for that.)