[executorch][llm] Add Buck targets for extension/llm/batching

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [executorch][muse] Move off-graph KV to the batching path and test batching in OSS CI #23375
• [executorch][muse] Add run_solo_batching #23374
• [executorch][muse] Add export_solo_batching.py #23373
• [executorch][cuda] Let CudaExecutor honor a program's prefill lower bound #23372
• [executorch][llm] Share the batching executors' session logic under extension/llm/batching/util #23371
• [executorch][cuda] Add CudaExecutor #23370
• [executorch][cuda] Add step_plan for the CUDA batching executor #23369
• -> [executorch][llm] Add Buck targets for extension/llm/batching #23368
• [executorch][cuda] Add CudaCellCache #23367
• [executorch][cuda] Lower off-graph KV in a cell layout #23366
• [executorch][cuda] Extract CudaKVPool from CudaSequenceKVCache #23365

extension/llm/batching builds only with CMake. Adds Buck targets mirroring
the CMake ones so fbcode code (the CUDA batching executor next in this stack)
can depend on it, and so its tests run in internal CI too:

• :batching: scheduler, executor seam, runner (extension_llm_batching)
• :module_executor: ModuleExecutor (extension_llm_batching_module)
• test:test: the scheduler, runner and prefix-cache tests
  (extension_llm_batching_test)

No source changes.

Differential Revision: D123051480

[pytorch-bot]

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