hotfix: bugfix to #756

[yzh119]

This pull request includes changes to the flashinfer/jit/__init__.py file to improve the import structure and handle the prebuilt_ops_uri import conditionally.

Improvements to import structure:

• Removed the unconditional import of prebuilt_ops_uri from aot_config and added it conditionally within the try-except block to handle cases where _kernels or _kernels_sm90 are not available. [1] [2]

cc @ByronHsu

[ByronHsu]

Thanks!