[Capture] Add error if qml.cond predicate is not a scalar

[albi3ro]

Context:

While updating catalyst tests to run with both frontends, I was encountering this test:

    def test_array_conversion_failed(self):
        """Test failure at converting array to bool using Autograph."""

        @qjit(autograph=True)
        def workflow(x):
            n = jnp.array([[1], [2]])

            if n:
                y = x**2
            else:
                y = 0

            return y

        with pytest.raises(
            TypeError, match="Array with multiple elements is not a valid predicate"
        ):
            workflow(3)

Which made me realize we don't actually have a similar verification for program capture. This adds that verification in.

Description of the Change:

Checks that all the predicates are scalars.

Benefits:

Enhanced verification.

Possible Drawbacks:

More stuff to run.

Related GitHub Issues:

[sc-97523]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.68%. Comparing base (9e9b4fd) to head (26cc307).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8066      +/-   ##
==========================================
- Coverage   99.68%   99.68%   -0.01%     
==========================================
  Files         543      543              
  Lines       56227    56229       +2     
==========================================
+ Hits        56052    56053       +1     
- Misses        175      176       +1     

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[SimoneGasperini]

Looks good! 😃