Fix unification of shapes

[momohatt]

This PR fixes the unification of shapes. Originally,

x = ShapeElem(1)
y = ShapeElem(None)
unify_shape((x,), (y,))

would have changed y into (ShapeElem(1),). However, this will result in the following inference.

x = torch.Tensor(4, 1, dtype=torch.float)
for i in range(5):
    x = x.expand(4, x.size(1) + 1)
# x : torch.Tensor(float32, (4, 1)) <-- shape should be (4, None)

This PR a) fixes this behavior, and b) adds implementation for applying shape annotation for tensors, which will be broken by a).

[shinh]

/test

[pfn-ci-bot]

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