Add falling back imports for brownian_lib

[dboyliao]

Add falling back import statements for brownian_lib module. Or tests may fail to run due to AttributeError or ImportError

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[dboyliao]

[dboyliao]

@googlebot I fixed it.

[patrick-kidger]

Googlebot doesn't seem convinced! Did you enter your GitHub user name when you signed the CLA?

In any case - this is only an issue for running the tests, correct? In which case I'd suggest that probably the better thing to do would be to modify the tests to skip brownian_lib if it doesn't exist. (Also I'd suggest working from the dev branch rather than the master branch.)

[lxuechen]

So the folder brownian_lib is to explicitly differentiate between the Python implementation and C++ implementation.

It doesn't really make sense to let brownian_lib import the same classes. I'd probably modify the tests with something like

if not hasattr(torchsde.brownian_lib, "BrownianPath"):
...

or

try:
    torchsde.brownian_lib.BrownianPath
except AttributeError:
...

Also, I appreciate the effort in sending in a PR, but I'd recommend opening up an issue first. It doesn't quite make sense to send in a PR before initiating any type of discussion.

[dboyliao]

@patrick-kidger Yes, I managed to get tests work with adding imports. I inspect the code of the tests and find out that BrownianPath and BrownianTree from both modules should work compatibly. That's why I come up with fix. I check the link googlebot gave me and it looks like I did sign the CLA before. Not sure why it does not work.

@lxuechen You are right, I should go for issue tracker first. Sorry for rushing an PR without discussion. Will close this one and open up an issue instead. Thanks for your suggestions.