bug: tensor types are not recognized as types

[justinchuby]

Seen in #223

1. "INT64" expects no type arguments, but 1 given [MYPY]

   def aten_ones(size: INT64[...]) -> TensorType:

2. Unexpected "..." [MYPY]

   def aten_row_stack(tensors: TensorType[...]) -> TensorType:

3. INT64 does not accept a default value Default value support in functions #229

   

[abock]

The bug here was that TensorType.__getitem__ was returning instances of a new TensorType to carry the shape information. Instead a new type needs to be returned. Should be fixed with #228.

[abock]

@justinchuby (3) should be filed as a separate issue please.