ClientError: An error occurred when calling the CreateEndpointConfig operation: MultiModel mode is not supported for instance type ml.g4dn.xlarge.

**Describe the bug**
Apparently, MultiModel mode is not supported in any of the GPU instance types. This is nowhere mentioned in the documentation.

**To reproduce**
```
create_endpoint_config_response = sm_client.create_endpoint_config(
EndpointConfigName = endpoint_config_name,
ProductionVariants=[{
'InstanceType': 'ml.g4dn.2xlarge',
'InitialInstanceCount': 1,
'InitialVariantWeight': 1,
'ModelName': model_name,
'VariantName': 'AllTraffic'}])
```

**System information**
A description of your system. Please provide:
- **SageMaker Python SDK version**: latest
- **Framework name (eg. PyTorch) or algorithm (eg. KMeans)**: Pytorch
- **Framework version**: 1.0
- **Python version**: 3.6
- **CPU or GPU**: GPU
- **Custom Docker image (Y/N)**: Y