[CircleCI] Update base images to ubuntu-2004 (#2343)

Summary:
Same change as done in this vision [PR](https://github.com/pytorch/vision/pull/5802)

As Ubuntu-1604 runners will no longer be available in early May
Update ubuntu-1604-cuda-10.1:201909-23 to ubuntu-2004-cuda-11.4:202110-01
Per [CircleCI Configuration reference](https://circleci.com/docs/2.0/configuration-reference/)

Resolves https://github.com/pytorch/audio/issues/2279

Pull Request resolved: https://github.com/pytorch/audio/pull/2343

Reviewed By: mthrok

Differential Revision: D35844880

Pulled By: atalman

fbshipit-source-id: 318a9fa42455e55664f3da6ab67625cb969f72e6

.circleci/config.yml

@@ -483,7 +483,7 @@ jobs:
   unittest_linux_gpu:
     <<: *binary_common
     machine:
-      image: ubuntu-1604-cuda-10.1:201909-23
+      image: ubuntu-2004-cuda-11.4:202110-01
     resource_class: gpu.nvidia.medium
     environment:
       <<: *environment

.circleci/config.yml.in

@@ -483,7 +483,7 @@ jobs:
   unittest_linux_gpu:
     <<: *binary_common
     machine:
-      image: ubuntu-1604-cuda-10.1:201909-23
+      image: ubuntu-2004-cuda-11.4:202110-01
     resource_class: gpu.nvidia.medium
     environment:
       <<: *environment