`training_data` in AutoMLConfig can not be a pandas data frame

I see the following exception when pandas data frame is passed as `training_data` in the [AutoMLConfig](https://docs.microsoft.com/en-us/python/api/azureml-train-automl-client/azureml.train.automl.automlconfig(class)?view=azure-ml-py#feedback).

``` text
ConfigException: ConfigException: Message: Input of type '' is not supported. Supported types: [azureml.data.tabular_dataset.TabularDataset]Please refer to documentation for converting to Supported types: https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.dataset.dataset?view=azure-ml-py InnerException: None ErrorResponse { "error": { "code": "UserError", "message": "Input of type '' is not supported. Supported types: [azureml.data.tabular_dataset.TabularDataset]Please refer to documentation for converting to Supported types: https://docs.microsoft.com/en-us/python/api/azureml-core/azureml.core.dataset.dataset?view=azure-ml-py", "details_uri": "https://aka.ms/AutoMLConfig", "target": "training_data", "inner_error": { "code": "BadArgument", "inner_error": { "code": "ArgumentInvalid", "inner_error": { "code": "InvalidInputDatatype" } } } } }
```

The documentation says that `training_data` can be a pandas data frame.

Versions:
- Python 3.6
- SDK version: 1.36.0

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* ID: be87853c-7d77-ec34-2bd2-a2f1e7104a3e
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* Content: [azureml.train.automl.AutoMLConfig class - Azure Machine Learning Python](https://docs.microsoft.com/en-us/python/api/azureml-train-automl-client/azureml.train.automl.automlconfig(class)?view=azure-ml-py)
* Content Source: [AzureML-Docset/stable/docs-ref-autogen/azureml-train-automl-client/azureml.train.automl.AutoMLConfig(class).yml](https://github.com/MicrosoftDocs/MachineLearning-Python-pr/blob/live/AzureML-Docset/stable/docs-ref-autogen/azureml-train-automl-client/azureml.train.automl.AutoMLConfig(class).yml)
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