fix: fix ensemble load error

[jung235]

To make Ensemble picklable, the second element of __reduce__ should be a tuple.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 56.71%. Comparing base (22e496a) to head (ce90c83).
Report is 14 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #9       +/-   ##
===========================================
+ Coverage   46.28%   56.71%   +10.43%     
===========================================
  Files          30       32        +2     
  Lines         955     1035       +80     
===========================================
+ Hits          442      587      +145     
+ Misses        513      448       -65     

Files	Coverage Δ
pydiffuser/tracer/ensemble.py	42.85% <0.00%> (+8.62%)	⬆️
pydiffuser/utils/config.py	67.44% <33.33%> (+12.17%)	⬆️

... and 6 files with indirect coverage changes