Update engine.py

Remove PP Grad Tail Check (until deepspeedai#2538 is merged to upstream)

Author: Quentin-Anthony

deepspeed/runtime/pipe/engine.py

@@ -990,9 +990,7 @@ def _exec_send_grads(self, buffer_id):
             if isinstance(inputs, tuple):
                 first_input = inputs[0]
                 assert all([torch.is_tensor(elt) for elt in inputs[1:]])
-                inputs_grad_tail = [
-                    elt.grad for elt in inputs[1:] if elt.grad is not None
-                ]
+                inputs_grad_tail = [elt.grad for elt in inputs[1:]]
             elif torch.is_tensor(inputs):
                 first_input = inputs
                 inputs_grad_tail = []