Fix requires_grad optional argument

[jwallwork23]

Closes #149.

Needed to switch requires_grad_opt -> requires_grad and use requires_grad_value to handle defaults (as with device_index).

Tested in my autograd example.

[jwallwork23]

Similarly for is_training.

[jwallwork23]

Oh and for rtol in the ResNet example, for consistency.

[jatkinson1000]

Thanks @jwallwork23
This is all sensible, thanks.

I suggest you add docs for these as part of #139 or similar.