[jax2tf] Enable bfloat16 tests (#4057)
* [jax2tf] Enable tests for bfloat16 FFT.
* Activated bfloat16 tests in test_unary_elementwise.
* Activated bfloat16/float16 tests for erf_inv.
* Made requested changes on the PR.
* Added documentation about why betainc doesn't work.
