Thanks for reporting this issue. I can reproduce it and it looks like a bug, as autograd seems to keep the gradients disabled even after leaving the no_grad() block:
with autocast(enabled=True):
    with torch.no_grad():
        y = net(input)

    z = net(input)
    print('z {}'.format(z.requires_grad))
> False

with autocast(enabled=False):
    with torch.no_grad():
        y = net(input)

    z = net(input)
    print('z {}'.format(z.requires_grad))
> True

@mcarilli Have you seen this behavior before?