Raise errors due to dimension mismatch issues in distributions

[neerajprad]

Most distributions do not explicitly check for correct dimensionality / broadcastable shapes when running their various methods. This is fine except that the client is forced to figure out what went wrong due to a some hard to comprehend error message. One simple thing to address in batch pdf would be this line, which is useful for a limited version of broadcasting but assumes that the data dimensions will always be greater than or equal to the parameter dimensions. Let us throw an error if that is not the case.

        if x.size != a.size():
            a = a.expand_as(x)
            b = b.expand_as(x)

[fritzo]

Fixed by #418, #419