[Fix] Fix duplicate register for searchers

[nijkah]

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Motivation

When flaml is not installed, it should be called to filter duplicated classes since SEARCH_ALG_IMPORT are function,

Check these

• https://github.com/ray-project/ray/blob/688e8a14f618cd7be8ad7975047285dc835480ca/python/ray/tune/search/__init__.py#L24-L33
• https://github.com/ray-project/ray/blob/688e8a14f6/python/ray/tune/search/flaml/flaml_search.py

Modification

Please briefly describe what modification is made in this PR.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@85e7ce2). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #116   +/-   ##
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  Coverage        ?   72.81%           
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  Files           ?       59           
  Lines           ?     1633           
  Branches        ?      236           
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  Hits            ?     1189           
  Misses          ?      345           
  Partials        ?       99           

Flag	Coverage Δ
unittests	72.81% <0.00%> (?)

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