Hi,
If you only want the gradient for the input, the simplest thing I can think of is:
model = # Your model
input = # Your input

# Make sure input requires grad
input.requires_grad_(True)

# Do the forward as usual
output = model(input)
loss = crit(output, label)

# Ask only for the grad wrt the input:
grad_input, = autograd.grad(loss, input)