Add docs for hls4ml Optimization API

docs/advanced/model_optimization.rst

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+========================
+hls4ml Optimization API
+========================
+
+Pruning and weight sharing are effective techniques to reduce model footprint and computational requirements. The hls4ml Optimization API introduces hardware-aware pruning and weight sharing.
+By defining custom objectives, the algorithm solves a Knapsack optimization problem aimed at maximizing model performance, while keeping the target resource(s) at a minimum. Out-of-the box objectives include network sparsity, GPU FLOPs, Vivado DSPs, memory utilization etc. 
+
+The code block below showcases three use cases of the hls4ml Optimization API - network sparsity (unstructured pruning), GPU FLOPs (structured pruning) and Vivado DSP utilization (pattern pruning). First, we start with unstructured pruning:
+
+.. code-block:: Python
+    from sklearn.metrics import accuracy_score
+    from tensorflow.keras.optimizers import Adam
+    from tensorflow.keras.metrics import CategoricalAccuracy
+    from tensorflow.keras.losses import CategoricalCrossentropy
+
+    from hls4ml.optimization.keras import optimize_model
+    from hls4ml.optimization.keras.utils import get_model_sparsity
+    from hls4ml.optimization.attributes import get_attributes_from_keras_model
+
+    from hls4ml.optimization.objectives import ParameterEstimator
+    from hls4ml.optimization.scheduler import PolynomialScheduler
+
+    # Define baseline model and load data
+    # X_train, y_train = ...
+    # X_val, y_val = ...
+    # X_test, y_test = ...
+    # baseline_model = ...
+
+    # Evaluate baseline model
+    y_baseline = baseline_model.predict(X_test)
+    acc_base = accuracy_score(np.argmax(y_test, axis=1), np.argmax(y_baseline, axis=1))
+    sparsity, layers = get_model_sparsity(baseline_model)
+    print(f'Baseline Keras accuracy: {acc_base}')
+    print(f'Baseline Keras sparsity, overall: {sparsity}')
+    print(f'Baseline Keras sparsity, per-layer: {layers}')
+
+    # Defining training parameters
+    # Epochs refers to the number of maximum epochs to train a model, after imposing some sparsity
+    # If the model is pre-trained, a good rule of thumb is to use between a 1/3 and 1/2 of the number of epochs used to train baseline model
+    epochs = 10
+    batch_size = 128
+    metric = 'accuracy'
+    optimizer = Adam()
+    loss_fn = CategoricalCrossentropy(from_logits=True)
+    
+    # Define the metric to monitor, as well as if its increasing or decreasing
+    # This disctinction allows us to optimize both regression and classification models
+    # In regression, e.g. minimize validation MSE & for classification e.g. maximize accuracy
+    metric, increasing = CategoricalAccuracy(), True
+
+    # Relative tolerance (rtol) is the the relative loss in metric the optimized model is allowed to incur
+    rtol = 0.975
+    
+    # A scheduler defines how the sparsity is incremented at each step
+    # In this case, the maximum sparsity is 50% and it will be applied at a polynomially decreasing rate, for 10 steps
+    # If the final sparsity is unspecified, it is set to 100%
+    # The optimization algorithm stops either when (i) the relative drop in performance is below threshold or (ii) final sparsity reached
+    scheduler = PolynomialScheduler(5, final_sparsity=0.5)
+
+    # Get model attributes
+    model_attributes = get_attributes_from_keras_model(baseline_model)
+    
+    # Optimize model
+    # ParameterEstimator is the objective and, in this case, the objective is to minimize the total number of parameters
+    optimized_model = optimize_model(
+        baseline_model, model_attributes, ParameterEstimator, scheduler,
+        X_train, y_train, X_val, y_val, batch_size, epochs, optimizer, loss_fn, metric, increasing, rtol 
+    )
+
+    # Evaluate optimized model
+    y_optimized = optimized_model.predict(X_test)
+    acc_optimized = accuracy_score(np.argmax(y_test, axis=1), np.argmax(y_optimized, axis=1))
+    sparsity, layers = get_model_sparsity(optimized_model)
+    print(f'Optimized Keras accuracy: {acc_optimized}')
+    print(f'Optimized Keras sparsity, overall: {sparsity}')
+    print(f'Opimized Keras sparsity, per-layer: {layers}')
+ 
+In a similar manner, it is possible to target GPU FLOPs or Vivado DSPs. However, in that case, sparsity is not equivalent to model sparsity. 
+Instead, it is the sparsity of the target resource. As an example: Starting with a network utilizing 512 DSPs and a final sparsity of 50%; the optimized network will use 256 DSPs.
+
+To optimize GPU FLOPs, the code is similar to above:
+
+.. code-block:: Python
+    from hls4ml.optimization.objectives.gpu_objectives import GPUFLOPEstimator
+    
+    # Optimize model
+    # Note the change from ParameterEstimator to GPUFLOPEstimator
+    optimized_model = optimize_model(
+        baseline_model, model_attributes, GPUFLOPEstimator, scheduler,
+        X_train, y_train, X_val, y_val, batch_size, epochs, optimizer, loss_fn, metric, increasing, rtol 
+    )
+
+    # Evaluate optimized model
+    y_optimized = optimized_model.predict(X_test)
+    acc_optimized = accuracy_score(np.argmax(y_test, axis=1), np.argmax(y_optimized, axis=1))
+    print(f'Optimized Keras accuracy: {acc_optimized}')
+
+    # Note the difference in total number of parameters
+    # Optimizing GPU FLOPs is equivalent to removing entire structures (filters, neurons) from the network
+    print(baseline_model.summary())
+    print(optimized_model.summary())
+
+Finally, optimizing Vivado DSPs is possible, given a hls4ml config:
+
+.. code-block:: Python
+    from hls4ml.utils.config import config_from_keras_model
+    from hls4ml.optimization.objectives.vivado_objectives import VivadoDSPEstimator
+    
+    # Create hls4ml config
+    default_reuse_factor = 4
+    default_precision = 'ac_fixed<16, 6>'
+    hls_config = config_from_keras_model(baseline_model, granularity='name', default_precision=default_precision, default_reuse_factor=default_reuse_factor)    
+    hls_config['IOType'] = 'io_parallel' 
+
+    # Optimize model
+    # Note the change from ParameterEstimator to VivadoDSPEstimator
+    optimized_model = optimize_model(
+        baseline_model, model_attributes, VivadoDSPEstimator, scheduler,
+        X_train, y_train, X_val, y_val, batch_size, epochs, optimizer, loss_fn, metric, increasing, rtol 
+    )