Allow passing optimizers to from_keras conversion function

[shelkesagar29]

_convert_common function in convert.py has optimizers parameter which allows user to select a subset of _optimizers from tf2onnx.optimizer while conversion is being done.

One way of using it to convert a TF saved model can be as follows.

from tf2onnx.optimizer import _optimizers
# Get all optimizers
updated_optimizers = copy.deepcopy(_optimizers)
# Remove two optimizers and then apply remaining optimizers during conversion
del updated_optimizers["q_dq_optimizer"]
del updated_optimizers["const_dequantize_optimizer"]

# 1. Load TensorFlow saved model
graph_def, inputs, outputs = tf_loader.from_saved_model(
    model_path=saved_model_path,
    input_names=None,
    output_names=None,
    tag="serve",
    signatures=["serving_default"]
)

# 2. Convert tf2onnx and save onnx file
model_proto, _ = convert._convert_common(
    graph_def,
    name=saved_model,
    opset=13,
    input_names=inputs,
    output_names=outputs,
    output_path=onnx_output_path,
    optimizers=updated_optimizers
)

However, currently there is no way to pass optimizers when using from_keras function to directly convert a keras model (without saving and loading). Converting from Keras directly is important since TF gives error many times for saving custom layers (this is especially true for QAT).

This PR adds optimizers parameter to from_keras function and passes it to _convert_common.

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[hwangdeyu]

Thanks for contribution!
Could you sign off every commit message following DCO required?

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[hwangdeyu]

LGTM.