[Bugfix][PD] Auto-clear producer KV cache if no pull notification (#2174)

### What this PR does / why we need it?

This PR addresses a critical issue where Node D (Device) failures cause
Node P (Processor) to hang due to inability to release KV cache.

**Trigger Scenarios:**  
1. Node D fails mid-inference (e.g., network disconnection)  
2. Node D rejects requests at a certain stage (e.g., via API server)  
3. Load-test script termination causes Node P or D to abort queued
requests

**Root Cause Analysis:**  
1. Currently, Node D sends a "KV cache pull complete, release approved"
message to Node P
2. This message is transmitted via the worker connector. If PD
connection breaks or requests are rejected upstream, Node D cannot send
the message
3. Node P will never release KV cache without receiving this message  

**Solution:**  
Following VLLM community's approach (NIXL connector timeout mechanism),
we're implementing:
- A timeout mechanism with comprehensive warnings  
- Updated README documentation  
- Reference: VLLM's optimization PR
[#20139](https://github.com/vllm-project/vllm/pull/20139)
### Does this PR introduce _any_ user-facing change?
None
### How was this patch tested?
None


- vLLM version: v0.10.2
- vLLM main:
9607d5eb44

---------

Signed-off-by: underfituu <hzhucong@163.com>

vllm_ascend/envs.py

@@ -159,11 +159,6 @@ env_variables: Dict[str, Callable[[], Any]] = {
     # caused by the initialization of the Mooncake connector.
     "PHYSICAL_DEVICES":
     lambda: os.getenv("PHYSICAL_DEVICES", None),
-    # Timeout (in seconds) for delayed KVCache block release. In the prefill
-    # node, if a request is marked for delayed KV block release and the blocks
-    # are not freed within this timeout, they will be forcibly released.
-    "VLLM_ASCEND_KVCACHE_DELAY_FREE_TIMEOUT":
-    lambda: int(os.getenv("VLLM_ASCEND_KVCACHE_DELAY_FREE_TIMEOUT", 250)),
 }
 
 # end-env-vars-definition
@@ -177,4 +172,4 @@ def __getattr__(name: str):
 
 
 def __dir__():
-    return list(env_variables.keys())
+    return list(env_variables.keys())