稀疏张量

Mars 支持稀疏张量，但目前只支持二维稀疏张量，未来将支持多维稀疏张量。

创建稀疏张量的函数

mars.tensor.tensor
mars.tensor.zeros	Return a new tensor of given shape and type, filled with zeros.
mars.tensor.eye	Return a 2-D tensor with ones on the diagonal and zeros elsewhere.
mars.tensor.identity	Return the identity tensor.
mars.tensor.random.randint	Return random integers from low (inclusive) to high (exclusive).