### What changes were proposed in this pull request?
1, update checking of numFeatures;
2, update `toString` to take `names` into account;
### Why are the changes needed?
1, should use `inputAttr.size` instead of `inputAttr.numAttributes` to get numFeatures;
2, this checking is necessary only if `$(indices).nonEmpty`, otherwise, `$(indices).max` will throw exception java.lang.UnsupportedOperationException: empty.max
3, in toString, should add length of names;
### Does this PR introduce _any_ user-facing change?
Yes, toString now count `names`;
### How was this patch tested?
existing testsuites
Closes#31354 from zhengruifeng/vector_slicer_clean.
Authored-by: Ruifeng Zheng <ruifengz@foxmail.com>
Signed-off-by: Ruifeng Zheng <ruifengz@foxmail.com>
e0251ac143