[Refactor] Cleanup platform (#5566)

### What this PR does / why we need it?
1. add `COMPILATION_PASS_KEY` constant
2. clean up useless platform interface `empty_cache`, `synchronize`,
`mem_get_info`, `clear_npu_memory`
3. rename `CUSTOM_OP_REGISTERED` to `_CUSTOM_OP_REGISTERED`
4. remove uesless env `VLLM_ENABLE_CUDAGRAPH_GC`

NPUPlatform is the interface called by vLLM. Do not call it inner
vllm-ascend.

### Does this PR introduce _any_ user-facing change?
This PR is just  a cleanup. All CI should pass.

### How was this patch tested?

- vLLM version: v0.13.0
- vLLM main:
7157596103

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm_ascend/compilation/compiler_interface.py

@@ -28,6 +28,7 @@ from torch.fx import GraphModule
 from vllm.compilation.compiler_interface import CompilerInterface
 
 from vllm_ascend.ascend_config import get_ascend_config
+from vllm_ascend.utils import COMPILATION_PASS_KEY
 
 
 def compile_fx(graph: GraphModule, example_inputs: list,
@@ -51,7 +52,7 @@ def fusion_pass_compile(
 ) -> tuple[Optional[Callable], Optional[Any]]:
 
     def compile_inner(graph, example_inputs):
-        current_pass_manager = compiler_config["graph_fusion_manager"]
+        current_pass_manager = compiler_config[COMPILATION_PASS_KEY]
         graph = current_pass_manager(graph, runtime_shape)
         return graph