Add Learning Rate Scheduler C API

[baijumeswani]

This pull request adds an API for registering a scheduler with the training session.

// c api
OrtTrainingSession* session;
g_ort_training_api->CreateTrainingSession(env, session_options, checkpoint_state,
                                          training_model.c_str(), eval_model.c_str(),
                                          optimizer_model.c_str(), &session));

// Set the initial learning rate.
g_ort_training_api->SetLearningRate(session, learning_rate);

// Register the linear leaning rate scheduler.
g_ort_training_api->RegisterLinearLRScheduler(session, warm_up_step_count, total_step_count);

// Take a scheduler step.
g_ort_training_api->SchedulerStep(session);