Improve ndarray input performance

[kazuki]

Classifier's partial_fit performance

• shape=(10000, 10), density=1.0

  • ndarray: 0.106 => 0.044
  • csr_matrix: 0.114 => 0.044

• shape=(10, 10000), density=1.0

  • ndarray: 0.071 => 0.068
  • csr_matrix: 0.072 => 0.068

• shape=(1000, 10000), density=0.001

  • ndarray: 0.037 => 0.017
  • csr_matrix: 0.018 => 0.007