Add epsilon-size override for ONNX exporter

I'll make a note to augment the ONNX exporter so it can handle this; this feels like it could end up in other places too.

```py3
def forward(self, obs: Tensor, epsilon: Tensor = None):
# we need to discard the extra dimensions of epsilon.
# the input epsilon is given for the original action space
# however, the policy outputs latent actions.
if epsilon is not None:
epsilon = epsilon[:, : self.latent_size]
```
_Originally posted by @tgolsson in https://github.com/EmbarkStudios/emote/pull/159#discussion_r1325550640_