[Feat](Mooncake) Supports multiple input suffixes for global_segment_size (#3690)

### What this PR does / why we need it?
- global_segment_size and local_buffer_size use constants for unified
management.
- Newly added support for input formats ending with GB, MB, KB, and B,
while being compatible with existing input methods.

### Does this PR introduce _any_ user-facing change?
- Users can use new input methods
- The documentation has also been modified

### How was this patch tested?


- vLLM version: v0.11.0
- vLLM main:
83f478bb19

---------

Signed-off-by: 李子琦 <liziqi_ing@163.com>

docs/source/user_guide/feature_guide/kv_pool_mooncake.md

@@ -5,7 +5,7 @@
 * Software:
   * Python >= 3.9, < 3.12
   * CANN >= 8.3.rc1
-  * PyTorch == 2.7.1, torch-npu == 2.7.1
+  * PyTorch >= 2.7.1, torch-npu >= 2.7.1.dev20250724
   * vLLM：main branch
   * vLLM-Ascend：main branch
   * Mooncake：main branch
@@ -41,7 +41,7 @@ The environment variable **MOONCAKE_CONFIG_PATH** is configured to the full path
     "use_ascend_direct": true,
     "alloc_in_same_node": true,
     "master_server_address": "xx.xx.xx.xx:50088",
-    "global_segment_size": 30000000000
+    "global_segment_size": "1GB" (1024MB/1048576KB/1073741824B/1073741824)
 }
 ```