Updates to get some of the unit tests within `backends/gpu/cpp_tests` working on the ROCm platform

[deven-amd]

This PR has a bunch of changes to get some of the unit tests within backends/gpu/cpp_tests working on the ROCm platform

Specifically the following unit-test will start passing on ROCm as a consequence of this PR

bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:gpu_types_test
bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/blas_wrapper_test
bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/dnn_wrapper_test
bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/fft_wrapper_test
bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/runtime_wrapper_test
bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/solver_wrapper_test

In addition the following test will compile (but hang at runtime)

bazel test --test_filter=*ROCm //backends/gpu/cpp_tests:wrapper/driver_wrapper_test

This PR includes ROCm specific updates to the above unit tests as well

The changes in this PR are broken down into functionally independent commits to help with the review.

[deven-amd]

/cc @chsigg @sanjoy

[chsigg]

Thanks a lot Deven, I only had time to skim through the changes so far, they look great. I will try to merge them tomorrow or early next week.