Array-like equality comparison for TransformedVariable

[st--]

Array-like behaviour for TransformedVariable.__eq__ and TransformedVariable.__ne__.
Resolves #1241.

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[st--]

It's clearly not quite as simple as changing this one line, but I'd really appreciate thoughts from the tensorflow-probability core developers as to what else might need changing to get the tests to pass and be able to close #1241!