Make MessagePassing interface thread-safe

[rusty1s]

Fixes #8994

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.26%. Comparing base (8d625c4) to head (0a1f522).

Files	Patch %	Lines
torch_geometric/nn/conv/message_passing.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9001      +/-   ##
==========================================
- Coverage   89.27%   89.26%   -0.01%     
==========================================
  Files         468      468              
  Lines       30011    30012       +1     
==========================================
- Hits        26791    26790       -1     
- Misses       3220     3222       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.