globalphase decomposes to nothing

[timmysilv]

Context:
GlobalPhase is a relatively new operator, and some plugins may not be able to handle it. To be consistent with the pre-GlobalPhase days, I'm making it decompose to nothing so devices will decompose it and make it disappear.

Description of the Change:
Implement GlobalPhase.compute_decomposition so it decomposes to nothing

Benefits:
Plugins won't break if they see it. It does have a matrix defined, but maybe some plugins need a whitelist of operators.

Possible Drawbacks:
Maybe plugins should fail if they see it, and maybe that's what we wanted?

Related GitHub Issues:
Good to merge with/before #4666
[sc-50428]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bea3682) 99.65% compared to head (acb11f5) 99.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4855      +/-   ##
==========================================
- Coverage   99.65%   99.64%   -0.01%     
==========================================
  Files         383      383              
  Lines       34513    34258     -255     
==========================================
- Hits        34393    34137     -256     
- Misses        120      121       +1     

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