Direct modules access when not tracing

[Xmaster6y]

What that this PR do?

Directly return the base model attributes when not tracing. (This is also how I understood the issue #156)

I propose to conditionally return and set attributes by checking the _tracer attribute of Envoy:

if self._envoy._tracer is None:
    return getattr(self._model, key)

return getattr(self._envoy, key)

PS

Adding envoys.append((self._module_path.lstrip("."), self)) is necessary to then manipulate modules like:

for name, module in model.named_modules():
    assert hasattr(model, name)

My bad for not including it in the first PR #165.