[Docs] xformers support

[iwr-redmond]

Testing by @DavidDragonsage here confirms that xformers installation is possible on supported platforms known to torchruntime.

The only known supported platform at the moment is cu124. However, rocm6.2.4 support will also be available if #9 is accepted.

It would be helpful if this extra feature is documented. The additional package can be installed from the command line with:

torchruntime install xformers

It can also be installed using the API with:

import torchruntime
torchruntime.install(["xformers"])

[iwr-redmond]

ROCm 6.3 also has an xformers build.

[iwr-redmond]

As of 12/27, the following Torch platforms have xformers support:

Platform	Status
cu128	✅
cu124	✅
rocm6.4	✅
rocm <= 6.2	❌ [1]
xpu	❌
cpu	✅
directml	❌ [2]

Notes:

1. See list here; 6.2.4 is supported but not enabled ([Feature] ROCm 6.2.4 support #9)
2. External to the Torch ecosystem