Add default values to for_loop

[obliviateandsurrender]

Context: This adds the default step=1 and a range-like syntax support for single-parameter.

Description of the Change: One can now do the following instead of specifying complete (0, 3, 1):

@for_loop(0, 3)
def f(i):
    qml.X(i)

@for_loop(3)
def f(i):
    qml.X(i)

Benefits:

Possible Drawbacks: None

Related GitHub Issues: [sc-70477] [sc-70478]

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (4934593) to head (50041c3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #6068    +/-   ##
========================================
  Coverage   99.66%   99.66%            
========================================
  Files         430      430            
  Lines       41786    41519   -267     
========================================
- Hits        41644    41379   -265     
+ Misses        142      140     -2     

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[albi3ro]

I'd be happy to approve once we get some more robust testing.

[albi3ro]

Thanks for the improved tests. Looks good to me 👍

[josh146]

Looks good from my end @obliviateandsurrender!