Fix None gradient to enable maximum-likelihood training. This fixes issue #59.

[yiboyang]

Fix None gradient to enable maximum-likelihood training.

This PR fixes resolves issue #59. Previously, likelihood-based training fails because the gradient of the log probability is None even when calling ODESolver.compute_likelihood with enable_grad=True. This PR fixes the issue by leaving out .detach() statements when enable_grad=True. The default behavior (with enable_grad=False) is unaffected.

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[timonpalm]

I can confirm that this fix worked. In the plot, you can see the likelihoods of a grid of points computed by the flow (A), their gradients using the current state of the repo (B) and the gradients with this fix.

It is apparent that (B) is clearly wrong.

Wish this PR to be merged soon. :)

[itaigat]

Thanks @yiboyang! I am happy to merge this; can you please remove the private folders from the notebook and solve the CI issues?

[yiboyang]

The formatting issue should be fixed now with 04e5165.