Fixes #4217

This PR breaks btrifact into btrifact and btrifact_with_info. The cwrap declarations for for tensor and variable are different in that the tensor.btrifact in TensorMath.cwrap still allows info optional argument, and var.btrifact in Decalaration.cwrap always sets info as NULL. In variable.py, a python method btrifact is added to support old info optional argument behavior, but I added deprecation warning in favor of btrifact_with_info.

I also modified preprocess_declarations.py's is_nullable for optional tensor arguments with NULL default values. An example of input declarations is the old btrifactin Declaration.cwrap:

  name: btrifact
  cname: btrifact
  types:
    - floating_point
  backends:
    - CPU
    - CUDA
  variants:
    - method
    - function
  return: argument 0,1
  arguments:
    - arg: THTensor* result
      output: True
    - arg: THIntegerTensor* pivots
      output: True
    - arg: THIntegerTensor* info
      kwarg_only: True
      default: NULL
    - arg: bool pivot
      kwarg_only: True
      default: "true"
    - THTensor* self
]]

, which gets parsed into the following:

{..., 'options': [{'arguments': [..., 
{'kwarg_only': True, 'default': None, 'type': 'THInteger Tensor*', 'name': 'info'}, 
...] ...}