Fix led torchscript

[LysandreJik]

LED can't run on torchscript:

Failure
Traceback (most recent call last):
  File "/home/xxx/transformers/tests/test_modeling_common.py", line 538, in _create_and_check_torchscript
    traced_model = torch.jit.trace(
  File "/home/xxx/transformers/.env/lib/python3.8/site-packages/torch/jit/_trace.py", line 735, in trace
    return trace_module(
  File "/home/xxx/transformers/.env/lib/python3.8/site-packages/torch/jit/_trace.py", line 952, in trace_module
    module._c._create_method_from_trace(
RuntimeError: 0INTERNAL ASSERT FAILED at "/pytorch/torch/csrc/jit/ir/alias_analysis.cpp":532, please report a bug to PyTorch. We don't have an op for aten::constant_pad_nd but it isn't a special case.  Argument types: Tensor, int[], bool, 

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/xxx/transformers/tests/test_modeling_led.py", line 284, in test_torchscript
    self._create_and_check_torchscript(config, inputs_dict)
  File "/home/xxx/transformers/tests/test_modeling_common.py", line 545, in _create_and_check_torchscript
    self.fail("Couldn't trace module.")
AssertionError: Couldn't trace module.