Replace instances of `torch.no_grad()` with `torch.inference_mode()` in evaluation reference scripts

[fmassa]

Since last release of PyTorch, it now provides a new context manager / function decorator that enables faster execution than torch.no_grad() https://pytorch.org/docs/stable/generated/torch.inference_mode.html?highlight=inference_mode#torch.inference_mode

We should replace instances of torch.no_grad() in our reference scripts whenever torch.inference_mode() is the correct candidate (for example, in evaluation code as in

vision/references/classification/train.py

Line 60 in 3d7244b

with torch.no_grad():

cc @datumbox

[XoMute]

Hi! I would like to try to work on this issue