deps(flashinfer): fix is_flashinfer_available() and make flashinfer optional dependency

[XuehaiPan]

Motivation

In the document, we need to install flashinfer manually after installing sglang.

Users cannot import SRT successfully if flashinfer is not installed.

Modifications

Return False in is_flashinfer_available() if flashinfer is not installed.

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.