## What changes were proposed in this pull request?
This patch adds a `compressed` method to ML `Matrix` class, which returns the minimal storage representation of the matrix - either sparse or dense. Because the space occupied by a sparse matrix is dependent upon its layout (i.e. column major or row major), this method must consider both cases. It may also be useful to force the layout to be column or row major beforehand, so an overload is added which takes in a `columnMajor: Boolean` parameter.
The compressed implementation relies upon two new abstract methods `toDense(columnMajor: Boolean)` and `toSparse(columnMajor: Boolean)`, similar to the compressed method implemented in the `Vector` class. These methods also allow the layout of the resulting matrix to be specified via the `columnMajor` parameter. More detail on the new methods is given below.
## How was this patch tested?
Added many new unit tests
## New methods (summary, not exhaustive list)
**Matrix trait**
- `private[ml] def toDenseMatrix(columnMajor: Boolean): DenseMatrix` (abstract) - converts the matrix (either sparse or dense) to dense format
- `private[ml] def toSparseMatrix(columnMajor: Boolean): SparseMatrix` (abstract) - converts the matrix (either sparse or dense) to sparse format
- `def toDense: DenseMatrix = toDense(true)` - converts the matrix (either sparse or dense) to dense format in column major layout
- `def toSparse: SparseMatrix = toSparse(true)` - converts the matrix (either sparse or dense) to sparse format in column major layout
- `def compressed: Matrix` - finds the minimum space representation of this matrix, considering both column and row major layouts, and converts it
- `def compressed(columnMajor: Boolean): Matrix` - finds the minimum space representation of this matrix considering only column OR row major, and converts it
**DenseMatrix class**
- `private[ml] def toDenseMatrix(columnMajor: Boolean): DenseMatrix` - converts the dense matrix to a dense matrix, optionally changing the layout (data is NOT duplicated if the layouts are the same)
- `private[ml] def toSparseMatrix(columnMajor: Boolean): SparseMatrix` - converts the dense matrix to sparse matrix, using the specified layout
**SparseMatrix class**
- `private[ml] def toDenseMatrix(columnMajor: Boolean): DenseMatrix` - converts the sparse matrix to a dense matrix, using the specified layout
- `private[ml] def toSparseMatrix(columnMajors: Boolean): SparseMatrix` - converts the sparse matrix to sparse matrix. If the sparse matrix contains any explicit zeros, they are removed. If the layout requested does not match the current layout, data is copied to a new representation. If the layouts match and no explicit zeros exist, the current matrix is returned.
Author: sethah <seth.hendrickson16@gmail.com>
Closes#15628 from sethah/matrix_compress.
e8810b73c4