linalg: Addition of LU decomposition

[Kriyszig]

Implementation based on http://www.engr.colostate.edu/~thompson/hPage/CourseMat/Tutorials/CompMethods/doolittle.pdf

Edit:

lu() can now take tensor of rank >=2 with last two dimension equal.

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[Kriyszig]

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[DirkToewe]

@Kriyszig Did You take a look at PR#1356? It also includes an LU implementation which includes:

• Backpropagation
• luSolve method
• Row Pivotization (drastically improves numeric stability)

[Kriyszig]

My bad, didn't see that!