Disable tests for mixed opt_levels, add bitwise accurate test of parameters

[ptrblck]

Increased atol to 1.5e-4 due to failing state_dict comparison for 'O0' and 'O2':

x = torch.tensor(0.8839)
torch.allclose(x.half().float(), x, atol=1e-4)
> False
torch.allclose(x.half().float(), x, atol=1.5e-4)
> True

[ptrblck]

First comment is deprecated.
The state_dict check now compares for bitwise accurate results and only for matching opt_level and restore_opt_level.