use_tracing=True was behaving unexpectedly because the handling of
single arguments was happening after the torch.jit.trace call.
This also fixes the check to specifically test for a torch.Tensor or
TensorPlaceholder so that both lists and tuples would be correctly
handled.
We do this by inroducing a TensorPlaceholder class, which can be used to
specify dynamic sizes. Internally, we canonicalize all example inputs
to TensorPlaceholder's.
This commit also adds some basic testing, which was missing before.
Sean Silva
powderluv