Add mixed integer precision test for op_mul

[swolchok]

This tests a possibly-surprising result: int8(100) * int8(100) with output type of long is 16 in ATen, even though the output type can hold 10000.

[kimishpatel]

wait what? I dont get why is it 16?

[manuelcandales]

wait what? I dont get why is it 16?

because 100 * 100 mod 256 = 16

this is how ATen works, because of functionalization
mul.out(x, y, out=out) must be equivalent to z = mul(x, y); out = z.to(out.dtype)
if x & y are int8, then z is also int8, and so multiplication turns into multiplication modulo 256
this is the downside of functionalization, it is sort of a compromise