op_input_variable: Create input for network
In Microsoft/CNTK-R: R Interface to the Microsoft Cognitive Toolkit (CNTK)

It creates an input in the network: a place where data, such as features and labels, should be provided.

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op_input_variable(shape, dtype = "float32", needs_gradient = FALSE,
  is_sparse = FALSE, dynamic_axes = c(get_default_batch_axis()),
  name = "")

`shape`	- list of ints representing tensor shape integer vector for dimensions of input tensor
`needs_gradient`	logical whether to conduct backprop on the tensor
`is_sparse`	logical whether variable is sparse
`dynamic_axes`	list of dynamic axis (only a single axis can be dynamic, i.e., either batch axis or time axis)