Add support of distance function and flexible reference index for `DISE`

[FanwangM]

The original implementation did not support the assigning distance function, and the assigning flexible index is not supported. This PR addresses these two problems.

This PR should be merged after #248 is merged, which will resolve the failing testing errors.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.01%. Comparing base (d931154) to head (07f8abe). Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
selector/methods/distance.py	92.40%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   96.00%   95.01%   -0.99%     
==========================================
  Files           9        9              
  Lines         975     1024      +49     
==========================================
+ Hits          936      973      +37     
- Misses         39       51      +12     

Files with missing lines	Coverage Δ
selector/methods/distance.py	95.12% <92.40%> (+2.52%)	:arrow_up:

... and 9 files with indirect coverage changes