addressing Torchscript compatibility

[ssandler-cat]

Making the code torch.jit.script() friendly; torch.jit.trace() is also supported.
See issues/83.

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[ssandler-cat]

If there is a use case to return unrefined_occlusion, unrefined_tracks and unrefined_expected_dist then the code should be modified as

    out = (torch.mean(torch.stack(trajectories.occlusion[p::p]), dim=0),       # occlusion
           torch.mean(torch.stack(trajectories.tracks[p::p]), dim=0),          # tracks
           torch.mean(torch.stack(trajectories.expected_dist[p::p]), dim=0),   # expected_dist
           trajectories.occlusion[:-1],                                        # unrefined_occlusion
           trajectories.tracks[:-1],                                           # unrefined_tracks
           trajectories.expected_dist[:-1]                                     # unrefined_expected_dist        
          )

having -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor]: replaced with -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor, list[torch.Tensor], list[torch.Tensor], list[torch.Tensor]]: in the forward annotation.

[yangyi02]

Thank you @SergeySandler for addressing the Torchscript compatibility issue here. Merging your code from Github backward into Google codebase is a little painful. But really appreciate your effort. We will see if we can do it from the Google side and push it forward to Github instead.