[mxpf8] Make mxfp8 dim1 cast kernel configurable

[danielvegamyhre]

Summary

• We recently developed a CUDA kernel in torchao to perform mxfp8 casting with scaling along dim1, which is ~1.4x faster than the previous Triton implementation, this results in e2e training speedup of 1.5% - 2.5% with torchtitan Llama3 8b with FSDP=4/8: Add CUDA kernel for MXFP8 dim1 casting ao#2513
• The integration into torchao is finished (integration of new mxfp8 casting cuda kernel ao#2564), so we need to update torchtitan to make the kernel choice for mxfp8 dim1 cast configurable to "triton", "cuda", or "torch".

Test plan

• Triton: NGPU=8 CONFIG_FILE="./torchtitan/models/llama3/train_configs/llama3_8b.toml" ./run_train.sh --training.steps=100 --model.converters="mx" --mx.recipe_name="mxfp8" --training.compile --mx.mxfp8_dim1_cast_kernel_choice="triton"
• Cuda: NGPU=8 CONFIG_FILE="./torchtitan/models/llama3/train_configs/llama3_8b.toml" ./run_train.sh --training.steps=100 --model.converters="mx" --mx.recipe_name="mxfp8" --training.compile --mx.mxfp8_dim1_cast_kernel_choice="cuda"

[danielvegamyhre]

cc @tianyu-l @vkuzo