[Usability]local_buffer_size support for units: GB, MB, KB, B (#4829)

What this PR does / why we need it?
Improve usability，local_buffer_size support for units: GB, MB, KB, B,
For example, "2GB"
{
    "local_hostname": "XXX.XXX.XXX.XXX",
    "metadata_server": "P2PHANDSHAKE",
    "protocol": "ascend",
    "device_name": "",
    "use_ascend_direct": true,
    "master_server_address": "XXX.XXX.XXX.XXX:50088",
    "global_segment_size": 60000000000,
    "local_buffer_size": "2GB"
}

Does this PR introduce any user-facing change?
local_buffer_size support for units: GB, MB, KB, B

How was this patch tested?
Mooncake configures local_buffer_size as GB, MB, KB, B
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: lty <linxianchong1@huawei.com>

vllm_ascend/distributed/kvpool/backend/mooncake_backend.py

@@ -94,8 +94,8 @@ class MooncakeStoreConfig:
             global_segment_size=_parse_global_segment_size(
                 config.get("global_segment_size",
                            DEFAULT_GLOBAL_SEGMENT_SIZE)),
-            local_buffer_size=(config.get("local_buffer_size",
-                                          DEFAULT_LOCAL_BUFFER_SIZE)),
+            local_buffer_size=_parse_global_segment_size(
+                config.get("local_buffer_size", DEFAULT_LOCAL_BUFFER_SIZE)),
             protocol=config.get("protocol", "tcp"),
             device_name=config.get("device_name", ""),
             master_server_address=config.get("master_server_address"))