### What changes were proposed in this pull request?
Sparse gemm use mothod `DenseMatrix.apply` to access the values, which can be optimized by skipping checking the bound and `isTransposed`
```
override def apply(i: Int, j: Int): Double = values(index(i, j))
private[ml] def index(i: Int, j: Int): Int = {
require(i >= 0 && i < numRows, s"Expected 0 <= i < $numRows, got i = $i.")
require(j >= 0 && j < numCols, s"Expected 0 <= j < $numCols, got j = $j.")
if (!isTransposed) i + numRows * j else j + numCols * i
}
```
### Why are the changes needed?
to improve performance, about 15% faster in the designed case
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
existing testsuite and additional performance test
Closes#32857 from zhengruifeng/gemm_opt_index.
Authored-by: Ruifeng Zheng <ruifengz@foxmail.com>
Signed-off-by: Ruifeng Zheng <ruifengz@foxmail.com>
5c96d643ee