[Misc] Add missing `_Backend` enums (#19081)

Signed-off-by: nicklucche <nlucches@redhat.com>

vllm/platforms/interface.py

@@ -47,6 +47,8 @@ class _Backend(enum.Enum):
     TORCH_SDPA = enum.auto()
     FLASHINFER = enum.auto()
     TRITON_MLA = enum.auto()  # Supported by V1
+    TRITON_MLA_VLLM_V1 = enum.auto()
+    FLASHMLA_VLLM_V1 = enum.auto()
     FLASHMLA = enum.auto()  # Supported by V1
     HPU_ATTN = enum.auto()
     PALLAS = enum.auto()