chore(dataflow): use n1 machine types for GPUs (GoogleCloudPlatform#6473)

## Description

Fixes GoogleCloudPlatform#6436

Explicitly use N1 machine types for GPU samples, it looks like the Dataflow service changed the default machine type to E2 shared-core machine types, which are not compatible with GPUs.

https://cloud.google.com/dataflow/docs/concepts/gpu-support#machine_types_specifications

## Checklist
- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: davidcavazos

dataflow/gpu-examples/pytorch-minimal/run.yaml

@@ -44,6 +44,7 @@ steps:
   - --job_name=$_JOB_NAME
   - --temp_location=$_TEMP_LOCATION
   - --sdk_container_image=gcr.io/$PROJECT_ID/$_IMAGE
+  - --machine_type=n1-standard-4
   - --experiment=worker_accelerator=type:$_GPU_TYPE;count:$_GPU_COUNT;install-nvidia-driver
   - --experiment=use_runner_v2
   - --disk_size_gb=50

dataflow/gpu-examples/tensorflow-landsat/run.yaml

@@ -47,6 +47,7 @@ steps:
   - --temp_location=$_TEMP_LOCATION
   - --worker_machine_type=custom-1-13312-ext
   - --sdk_container_image=gcr.io/$PROJECT_ID/$_IMAGE
+  - --machine_type=n1-standard-4
   - --experiment=worker_accelerator=type:$_GPU_TYPE;count:$_GPU_COUNT;install-nvidia-driver
   - --experiment=use_runner_v2
   - --experiment=no_use_multiple_sdk_containers

dataflow/gpu-examples/tensorflow-minimal/run.yaml

@@ -44,6 +44,7 @@ steps:
   - --job_name=$_JOB_NAME
   - --temp_location=$_TEMP_LOCATION
   - --sdk_container_image=gcr.io/$PROJECT_ID/$_IMAGE
+  - --machine_type=n1-standard-4
   - --experiment=worker_accelerator=type:$_GPU_TYPE;count:$_GPU_COUNT;install-nvidia-driver
   - --experiment=use_runner_v2
   - --experiment=no_use_multiple_sdk_containers