I had also tried this. it gives the same result. For tensor it gives 1 and for Variable the error.
>>> var.norm(p=float("inf"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/torch3/lib/python3.5/site-packages/torch/autograd/variable.py", line 396, in norm
    return super(Variable, self).norm(p)
RuntimeError: value cannot be converted to type float without overflow: inf
>>> tor.norm(p=float("inf"))
1.0