Add MPI extension with Allreduce! forward rule

[lcw]

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diff --git a/test/integration/MPI/runtests.jl b/test/integration/MPI/runtests.jl
index 4bce80bf..ee9b8c0f 100644
--- a/test/integration/MPI/runtests.jl
+++ b/test/integration/MPI/runtests.jl
@@ -3,5 +3,5 @@ using Enzyme
 using Test
 
 @testset "collectives" for np in (1, 2, 4)
-        run(`$(mpiexec()) -n $np $(Base.julia_cmd()) --project=$(@__DIR__) $(joinpath(@__DIR__, "collectives.jl"))`)
+    run(`$(mpiexec()) -n $np $(Base.julia_cmd()) --project=$(@__DIR__) $(joinpath(@__DIR__, "collectives.jl"))`)
 end

[vchuravy]

@lcw and I also realized that this may not be needed for the application we look at. (We may only need adjoint MPI_Allreduce)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 69.95%. Comparing base (97dbf1b) to head (839c6e0). :warning: Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/EnzymeMPIExt.jl	0.00%	14 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2745      +/-   ##
==========================================
- Coverage   70.00%   69.95%   -0.06%     
==========================================
  Files          58       59       +1     
  Lines       19295    19309      +14     
==========================================
  Hits        13507    13507              
- Misses       5788     5802      +14     

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