Create add methods on initializers and attributes

[justinchuby]

1. It's now possible to do node.attributes.add(ir.Attr(...)) to avoid having to get the name as a redundant step.

2. Also allow node attributes to be initialized with a dictionary

   • This way users do not have to use

   ir.Node(attributes=node.attributes.values())

   when copying the node. Instead, it is possible to

   ir.Node(attributes=node.attributes)

3. Implement get_* methods on attributes to make getting default attributes easier.

   Before

   attr = node.attributes.get("attr")
   if attr is not None:
       value = attr.as_int()
   else:
       value = 42

   now

   value = node.attributes.get_int("attr", 42)

[codecov]

Codecov Report

Attention: Patch coverage is 95.58011% with 8 lines in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (d28c4e6) to head (1392da0).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onnx_ir/_graph_containers.py	91.54%	6 Missing ⚠️
src/onnx_ir/_core.py	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   81.75%   82.23%   +0.48%     
==========================================
  Files          57       57              
  Lines        6965     7110     +145     
  Branches      948      958      +10     
==========================================
+ Hits         5694     5847     +153     
+ Misses        905      902       -3     
+ Partials      366      361       -5     

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