[mlir][python] Fix how the mlir variadic Python accessor _ods_equally_sized_accessor is used (#101132) (#106003)

As reported in llvm/llvm-project#101132, this
fixes two bugs:

1. When accessing variadic operands inside an operation, it must be
accessed as `self.operation.operands` instead of `operation.operands`
2. The implementation of the `equally_sized_accessor` function is doing
wrong arithmetics when calculating the resulting index and group sizes.

I have added a test for the `equally_sized_accessor` function, which did
not have a test previously.

Author: kasper0406

mlir/python/mlir/dialects/_ods_common.py

@@ -51,21 +51,22 @@ def segmented_accessor(elements, raw_segments, idx):
 
 
 def equally_sized_accessor(
-    elements, n_variadic, n_preceding_simple, n_preceding_variadic
+    elements, n_simple, n_variadic, n_preceding_simple, n_preceding_variadic
 ):
     """
     Returns a starting position and a number of elements per variadic group
     assuming equally-sized groups and the given numbers of preceding groups.
 
       elements: a sequential container.
+      n_simple: the number of non-variadic groups in the container.
       n_variadic: the number of variadic groups in the container.
       n_preceding_simple: the number of non-variadic groups preceding the current
           group.
       n_preceding_variadic: the number of variadic groups preceding the current
           group.
     """
 
-    total_variadic_length = len(elements) - n_variadic + 1
+    total_variadic_length = len(elements) - n_simple
     # This should be enforced by the C++-side trait verifier.
     assert total_variadic_length % n_variadic == 0