For example:
a = torch.Tensor([1, 2, 3])
b = torch.Tensor([[1, 2, 3]])
b@b.t()
Out[4]: 
 14
[torch.FloatTensor of size 1x1]
a@a.t()
Traceback (most recent call last):
  File "/Users/omz/anaconda3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2862, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-5-e27dc62b681a>", line 1, in <module>
    a@a.t()
RuntimeError: t() expects a 2D tensor, but self is 1D