[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/device_allocator/camem.py

@@ -25,8 +25,6 @@ import torch
 from acl.rt import memcpy  # type: ignore # noqa: F401
 from vllm.logger import logger
 
-from vllm_ascend.platform import NPUPlatform
-
 
 def find_loaded_library(lib_name) -> Optional[str]:
     """
@@ -196,11 +194,10 @@ class CaMemAllocator:
             handle = data.handle
             if data.tag in offload_tags:
                 size_in_bytes = handle[1]
-                cpu_backup_tensor = torch.empty(
-                    size_in_bytes,
-                    dtype=torch.uint8,
-                    device='cpu',
-                    pin_memory=NPUPlatform.is_pin_memory_available())
+                cpu_backup_tensor = torch.empty(size_in_bytes,
+                                                dtype=torch.uint8,
+                                                device='cpu',
+                                                pin_memory=True)
                 cpu_ptr = cpu_backup_tensor.data_ptr()
                 ACL_MEMCPY_DEVICE_TO_HOST = 2
                 dest_max = cpu_ptr + size_in_bytes * 2