fix: fix issue with same doc type in spawned processes

[JoanFM]

When multiple Executors inside a Flow have the same model and they are used as docker containers or using JINA_MP_START_METHOD=spawn there will be an exception when served via HTTP.

This PR fixes this issue by making sure that only one model instance is considered for the same model name.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (67c83c2) 76.05% compared to head (1c4b9d3) 75.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6062      +/-   ##
==========================================
- Coverage   76.05%   75.94%   -0.12%     
==========================================
  Files         145      145              
  Lines       13916    13925       +9     
==========================================
- Hits        10584    10575       -9     
- Misses       3332     3350      +18     

Flag	Coverage Δ
jina	75.94% <30.00%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ve/runtimes/gateway/http_fastapi_app_docarrayv2.py	0.00% <0.00%> (ø)
jina/serve/stream/__init__.py	84.65% <37.50%> (-2.85%)	⬇️

... and 4 files with indirect coverage changes

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