Ensured that dtype subclasses are hashable

[superbobry]

Python data model requires a class to implement both __eq__ and
__hash__ to be considered hashable. If a class only implements
__eq__, it gets an auto-generated __hash__ = None, which makes
it non-hashable.

So, as things stand, tl.pointer_type and tl.block_type instances fail
to hash. The fix is to define __hash__ explicitly in the class body, as
suggested in *:

If a class that overrides __eq__ needs to retain the implementation of
__hash__ from a parent class, the interpreter must be told this explicitly
by setting __hash__ = <ParentClass>.__hash__.

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[Moerafaat]

@ThomasRaoux can you please take a look at this fix? Thanks!

[pawelszczerbuk]

Wouldn't this map scalar type and tensor with elements of the same type (like float32 and tensor of float32) and also different shape tensors to be the same? I would think we should include shape in the hash.