Pytorch: Add logical AND/OR/NOT/XOR operations

[fmassa]

Not super important, but might be useful to have those logical operators in the core.

As a workaround implementation, they can be simulated with current functions. For example

-- t1 and t2 are ByteTensors with 0 or 1 only
function tensor_and(t1, t2)
  return (t1 + t2):eq(2)
end

[apaszke]

Implemented in #342.