BugFix: Resolve PolicyFlashlb warm up function attribute error (#4741)

## Description
Fix the AttributeError caused by incorrect invocation of the warm-up
function in the FlashLB algorithm:
1. **Root Cause**: The warm-up function for FlashLB is defined outside
the `PolicyFlashlb` class (not a class method), but the code incorrectly
attempted to call it via the `PolicyFlashlb` class instance.
2. **Key Fix**: Clarify the invocation rule for FlashLB: when selecting
the FlashLB algorithm, the warm-up function must be called in advance to
precompile and warm up the algorithm (invoked as a standalone function),
instead of calling it through the `PolicyFlashlb` class.
3. **Impact**: Resolve the runtime error when using FlashLB, ensure the
algorithm pre-compilation/warm-up process works as expected, and avoid
attribute missing exceptions.

Signed-off-by: Mercykid-bash <ruanche0218@gmail.com>

vllm_ascend/eplb/core/policy/policy_factory.py

@@ -3,7 +3,7 @@
 from .policy_abstract import DynamicConfig, EplbPolicy
 from .policy_dynamic_ep import DynamicEplb
 from .policy_dynamic_ep_v2 import DynamicEplbV2
-from .policy_flashlb import FlashLB
+from .policy_flashlb import FlashLB, warm_up
 from .policy_random import RandomLoadBalance
 
 
@@ -29,5 +29,5 @@ class PolicyFactory:
         policy_class = policy.get(policy_type, RandomLoadBalance)
         policy_instance = policy_class(config)
         if policy_type == 3:
-            policy_instance.warm_up()
-        return policy_instance
+            warm_up()
+        return policy_instance