[Bug] NemotronH config export fails: num_hidden_layers not found

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Bug

NemotronH config export crashes before ONNX generation. NemotronHConfig uses layers_block_type (list of layer type strings) instead of num_hidden_layers. _export_hybrid_mamba_config treats num_hidden_layers as required.

KeyError: "Required field 'num_hidden_layers' not found in config"

Fix: derive from layers_block_type when absent.

tensorrt_edgellm/onnx_export/config_export.py, in _export_hybrid_mamba_config:

if "num_hidden_layers" not in config_dict and "layers_block_type" in config_dict:
    config_dict = dict(config_dict)
    config_dict["num_hidden_layers"] = len(config_dict["layers_block_type"])