Converts a nest of TensorSpecs to a nest of matching placeholders.

nested_tensor_specs A nest of tensor specs.
default Optional constant value to set as a default for the placeholder.
name_scope String name for the scope to create the placeholders in.
outer_dims Optional leading dimensions for the placeholder.

A nest of placeholders matching the given tensor spec.

ValueError If a default is provided outside of the allowed types, or if default is a np.array that does not match the spec shape.