Set input and output size for a model, and query if they've been adequately set.

[adamhutchings]

This is to implement the functions get_input_size, get_output_size, set_input_size, set_output_size,and is_specified.

As per the API document:

get_input_size
Scope: jml::Model::
Arguments:
Expected constraints:
Returns:
unsigned
Description:
This function queries the input size for a model.

get_output_size
Scope: jml::Model::
Arguments:
Expected constraints:
Returns:
unsigned
Description:
This function queries the output size for a model.

set_input_size
Scope: jml::Model::
Arguments:
unsigned size
Expected constraints:
size > 0
Returns: void
Description:
This function sets the input size for a model. This function errors if the model already has attached layers or training data.

set_output_size
Scope: jml::Model::
Arguments:
unsigned size
Expected constraints:
size > 0
Returns: void
Description:
This function sets the output size for a model. This function errors if the model already has attached layers or training data.

is_specified
Scope: jml::Model::
Arguments:
Expected constraints:
Returns:
bool
Description:
This function queries whether a given model is fully specified, and ready to be given training data and layers. This should return true if input and output size have been specified, and false otherwise.