max_concurrent_iterations should default to all nodes of the target cluster

A user submitting a parallelizable job like an AutoML run to a compute cluster wants to make use of all nodes in the cluster.
But by default AutoMLConfig defaults to max_concurrent_iterations=1. This is the opposite of the behavior expected by users, and contradicts the default AutoML behavior when configuring a run via the ml.azure.com UI, which defaults to using all nodes of the target compute.

More expected behavior would be default to using all nodes in target compute cluster, along the lines of

max_concurrent_iterations=-1

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#### Document Details

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* ID: 0bc2b21e-6b1a-cb94-2857-147177a29d7c
* Version Independent ID: d14620a6-a2f6-49f1-632e-73903d41de8c
* Content: [azureml.train.automl.automlconfig.AutoMLConfig class - Azure Machine Learning Python](https://docs.microsoft.com/en-us/python/api/azureml-train-automl-client/azureml.train.automl.automlconfig.automlconfig?view=azure-ml-py)
* Content Source: [AzureML-Docset/stable/docs-ref-autogen/azureml-train-automl-client/azureml.train.automl.automlconfig.AutoMLConfig.yml](https://github.com/MicrosoftDocs/MachineLearning-Python-pr/blob/live/AzureML-Docset/stable/docs-ref-autogen/azureml-train-automl-client/azureml.train.automl.automlconfig.AutoMLConfig.yml)
* Service: **machine-learning**
* Sub-service: **core**
* GitHub Login: @DebFro
* Microsoft Alias: **debfro**

Revise a referência de AutoMLConfig vinculada em AzureML-Docset/stable/docs-ref-autogen/azureml-train-automl-client/azureml.train.automl.automlconfig.AutoMLConfig.yml e localize a implementação de max_concurrent_iterations. Confirme como -1 é interpretado para um cluster de computação de destino, depois atualize o valor padrão para que os trabalhos paralelizáveis usem todos os nós disponíveis e verifique o comportamento documentado.

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