[Tensor] Disambiguate undefined Tensor dimensions from Scalar Tensor

Define a Tensor with undefined dims by explicitly setting mSize to 0.
Provide undefined() method which must be used instead of empty()
to test whether a Tensor as associated dimensions and can be used
for forwarding dimensions downward.
This disambiguates from defined scalar Tensor which actually have
empty dimensions, but size of 1.
Note that as soon as a Tensor with undefined dimensions is resized,
it's dimensions are permanently defined (though may still change).

This change solves issues in operators forwardDims() when
input values are scalar, i.e. !undefined() and empty().