Fix batched matmul & add a unit test

[chhwang]

No description provided.

[codecov]

Codecov Report

Merging #117 (042f7a8) into main (faa69f5) will decrease coverage by 0.01%.
The diff coverage is 75.92%.

❗ Current head 042f7a8 differs from pull request most recent head e8ddc44. Consider uploading reports for the commit e8ddc44 to get more accurate results

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
- Coverage   73.39%   73.39%   -0.01%     
==========================================
  Files         112      112              
  Lines        9039     9072      +33     
==========================================
+ Hits         6634     6658      +24     
- Misses       2405     2414       +9     

Files Changed	Coverage Δ
ark/ops/ops_matmul_test.cu	86.25% <75.92%> (-2.12%)	⬇️

... and 1 file with indirect coverage changes