Set running_torchscript recursively

[carmocca]

What does this PR do?

Fixes bug reported in Slack:

Traceback (most recent call last):
  File "t.py", line 23, in <module>
    lm.to_torchscript(method='script')
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/autograd/grad_mode.py", line 27, in decorate_context
    return func(*args, **kwargs)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/pytorch_lightning/core/module.py", line 1926, in to_torchscript
    torchscript_module = torch.jit.script(self.eval(), **kwargs)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_script.py", line 1265, in script
    return torch.jit._recursive.create_script_module(
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 451, in create_script_module
    concrete_type = get_module_concrete_type(nn_module, share_types)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 402, in get_module_concrete_type
    concrete_type = concrete_type_store.get_or_create_concrete_type(nn_module)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 343, in get_or_create_concrete_type
    concrete_type_builder = infer_concrete_type_builder(nn_module)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 208, in infer_concrete_type_builder
    sub_concrete_type = get_module_concrete_type(item, share_types)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 402, in get_module_concrete_type
    concrete_type = concrete_type_store.get_or_create_concrete_type(nn_module)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 343, in get_or_create_concrete_type
    concrete_type_builder = infer_concrete_type_builder(nn_module)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 251, in infer_concrete_type_builder
    overloads.update(get_overload_name_mapping(get_overload_annotations(nn_module, ignored_properties)))
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/torch/jit/_recursive.py", line 617, in get_overload_annotations
    item = getattr(mod, name, None)
  File "/home/pete/.pyenv/versions/3.8.6/envs/python38/lib/python3.8/site-packages/pytorch_lightning/core/module.py", line 180, in trainer
    raise RuntimeError(f"{self.__class__.__qualname__} is not attached to a `Trainer`.")
RuntimeError: Child is not attached to a `Trainer`.

Does your PR introduce any breaking changes? If yes, please list them.

None

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