Model is constrained to always predicting a function call only.
If "allowed_function_names" are set, the predicted function call will be
limited to any one of "allowed_function_names", else the predicted
function call will be any one of the provided "function_declarations".
Auto
Default model behavior, model decides to predict either a function call
or a natural language response.
None
Model will not predict any function call. Model behavior is same as when
not passing any function declarations.
Unspecified
Unspecified function calling mode. This value should not be used.