When timeout connecting, you get URLError: <urlopen error timed out>, In that case, build it from source.

Set `packaging` and `ninja` as build time dependencies rather than runtime dependencies.

All integer parameters are specialized by default, so the two parameters
removed in this commit could lead to kernel re-compilation, even if
they were completely unused.

* Enable paged attention in varlen forward

* Format + fix padding

Optimize compile to 1: avoid oom 2: minimize swap usage 3: avoid threads starvation when letting ninja decide how many workers to spawn or manual MAX_JOBS "guesses". Logic is to take the min value of MAX_JOBS auto-calculated by two metrics: 1: cpu cores 2: free memory. This should allow flash-attn to compile close to the most efficient manner under any consumer/server env. (#832)