bug: explain is not right after enable_distributed_eval_index enabled

[BohuTANG]

Summary

table: db7861.t7861 (this table segment numbers same as block numbers):

mysql> select * from fuse_snapshot('db7861', 't7861') limit 1\G;
*************************** 1. row ***************************
         snapshot_id: d655707aba6e43af9d13d2e7ed0774ed
   snapshot_location: 112/151214/_ss/d655707aba6e43af9d13d2e7ed0774ed_v1.json
      format_version: 1
previous_snapshot_id: 7c18aa03089940d4800deb45e91f29cb
       segment_count: 21856
         block_count: 21856
           row_count: 170750000
  bytes_uncompressed: 212278377285
    bytes_compressed: 66885815033
          index_size: 164959939
           timestamp: 2022-10-08 07:50:02.222926

enable_distributed_eval_index=1(default)

mysql> explain select * from db7861.t7861 where c24='0.10437281852861613';
+-------------------------------------------------------------------------------+
| explain                                                                       |
+-------------------------------------------------------------------------------+
| TableScan                                                                     |
| ├── table: default.db7861.t7861                                               |
| ├── read rows: 170750000                                                      |
| ├── read bytes: 66885815033                                                   |
| ├── partitions total: 21856  -- this is the segments number  (this is not right)       
| ├── partitions scanned: 21856     -- this is the segments number  (this is not right)                               
| └── push downs: [filters: [(c24 = '0.10437281852861613')], limit: NONE]       |
+-------------------------------------------------------------------------------+

enable_distributed_eval_index=0

mysql> set enable_distributed_eval_index=0;
Query OK, 0 rows affected (0.01 sec)

mysql> explain select * from db7861.t7861 where c24='0.10437281852861613';
+-------------------------------------------------------------------------------+
| explain                                                                       |
+-------------------------------------------------------------------------------+
| TableScan                                                                     |
| ├── table: default.db7861.t7861                                               |
| ├── read rows: 5323985                                                        |
| ├── read bytes: 2083488768                                                    |
| ├── partitions total: 21856            -- this is block number(it's ok)
| ├── partitions scanned: 683          -- this is block number after pruned (it's ok)
| └── push downs: [filters: [(c24 = '0.10437281852861613')], limit: NONE]       |
+-------------------------------------------------------------------------------+

[BohuTANG]

We should back the explain to blocks for partitions not segments, cc @zhang2014