[Bug]: Hybrid search with expression template does not work

[xiaocai2333]

Is there an existing issue for this?

• I have searched the existing issues

Environment

- Milvus version: master & 2.4
- Deployment mode(standalone or cluster):
- MQ type(rocksmq, pulsar or kafka):    
- SDK version(e.g. pymilvus v2.0.0rc2):
- OS(Ubuntu or CentOS): 
- CPU/Memory: 
- GPU: 
- Others:

Current Behavior

test example:

{
  "collection_name": "collection_rxobtrdg",
  "requests": [
    {
      "collection_name": "collection_rxobtrdg",
      "partition_names": [],
      "output_fields": ["*"],
      "nq": 1,
      "dsl": "varChar == {value}",
      "dsl_type": 1,
      "placeholder_group": {
        "type": "Buffer",
        "data": [
          10, 40, 10, 2, 36, 48, 16, 101, 26, 32, 0, 0, 128, 63, 0, 0, 0, 64, 0,
          0, 64, 64, 0, 0, 128, 64, 0, 0, 160, 64, 0, 0, 192, 64, 0, 0, 224, 64,
          0, 0, 0, 65
        ]
      },
      "search_params": [
        { "key": "anns_field", "value": "vector" },
        { "key": "params", "value": "{\"nprobe\":2}" },
        { "key": "topk", "value": 1 },
        { "key": "offset", "value": 0 },
        { "key": "metric_type", "value": "" },
        { "key": "ignore_growing", "value": false }
      ],
      "consistency_level": "Strong",
      "expr_template_values": { "value": { "string_val": "d" } }
    }
  ],
  "rank_params": [
    { "key": "strategy", "value": "rrf" },
    { "key": "params", "value": "{\"k\":60}" },
    { "key": "round_decimal", "value": -1 },
    { "key": "limit", "value": 1 }
  ],
  "output_fields": ["*"],
  "consistency_level": "Strong"
}

return error:

failed to create query plan: the value of expression template variable name {value} is not found: invalid parameter

Expected Behavior

search successfully.

Steps To Reproduce

No response

Milvus Log

No response

Anything else?

No response

[xiaocai2333]

/assign
/unassign @yanliang567