[XLA:GPU] Implement new Triton Softmax codegen based on symbolic tiles and indexing maps.
[XLA:GPU] Implement new Triton Softmax codegen based on symbolic tiles and indexing maps.

Symbolic tiles are still under development and can process all instructions. The new codegen is used only when all instruction in the fusion are supported, otherwise we fall back to the old codegen.

Triton Softmax codegen is behind `--xla_gpu_enable_triton_softmax_fusion` flag that is not turned on by default, so this change shouldn't affect any production workloads.
