fix avoid re-defining the differentiation objective to support AD pre-compilation

[Red-Portal]

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[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.05%. Comparing base (538bafe) to head (0049369).
Report is 10 commits behind head on master.

❗ Current head 0049369 differs from pull request most recent head ef3c312

Please upload reports for the commit ef3c312 to get more accurate results.

Files	Patch %	Lines
src/objectives/elbo/repgradelbo.jl	70.58%	5 Missing ⚠️
src/AdvancedVI.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   93.33%   88.05%   -5.28%     
==========================================
  Files          11       11              
  Lines         195      201       +6     
==========================================
- Hits          182      177       -5     
- Misses         13       24      +11     

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