refactor: simplify inference engine and backend dispatch

- merge _generate_streaming/_generate_non_streaming into single _generate() with stream flag
- delete dead GenerationRequest class and generate_with_request method
- inline _next_token helper into generate_async
- replace flash-attn double-checked locking with functools.lru_cache
- extract _write_and_gather_kv helper shared by TorchNative/FlashAttn backends
- inline _kv_cache_is_contiguous into its sole call site in FlashAttnBackend
- change default backend priority from flash>cuda>torch to cuda>flash>torch
- add ASTR_BACKEND env var to override default backend at resolve time
- add supports_graph() static method to AttentionBackend ABC, override in CudaBackend
- replace isinstance(get_backend(), CudaBackend) with get_backend().supports_graph() in executor
- add torch.cuda.is_available() guard to CudaBackend.supports()

tests/extension/test_backend.py

@@ -19,12 +19,13 @@ def test_default_backend_is_torch_native():
     """Default is the highest-priority available backend (flash > cuda > torch)."""
     from astrai.extension.attention_backend import (
         CudaBackend,
+        FlashAttnBackend,
         TorchNativeBackend,
         _resolve_default_backend,
     )
 
     backend = get_backend()
-    assert isinstance(backend, (CudaBackend, TorchNativeBackend))
+    assert isinstance(backend, (CudaBackend, FlashAttnBackend, TorchNativeBackend))
     assert isinstance(backend, type(_resolve_default_backend()))