Improve test_tt_TTOI test

[JeanKossaifi]

Thanks for adding the TT-TTOI @Lili-Zheng-stat. I fixed the test for MXNet but also realized the test setting is a little strange, the rank is just 1? I think it would make more sense to create a random tensor (with random.random_tt) without noise and check that the relative reconstruction error is lower than a fixed tolerance.

The test also randomly fails with various backends (mostly with dtype float32).