Make call operator on module holder call forward (#15831)

Make call operator on module holder call forward (#15831)

Summary:
In Python, you can use the call operator to invoke the `forward()` method of a module. In C++ this was currently not possible, because I couldn't figure out how to deduce the return type of a module's `forward()` method under the constraint that `forward()` may not exist at all (since the base module class in C++ does not mandate a `forward()` method). I now figured it out, so the call operator can be used.

ezyang ebetica
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15831

Differential Revision: D13652676

Pulled By: goldsborough

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