Refresh backend README with current progress (#20356) #14106

Job	Run time
CI run decision / decide	30s
Get changed files / get-changed-files	3s
test-executorch-cuda-build-${{ matrix.cuda-version }}	-1s
check-all-cuda-builds	-1s
test-models-cuda	-1s
export-model-cuda-artifact	-1s
unittest-cuda	-1s
test-cuda-pybind	-1s
test-model-cuda-e2e	-1s
	26s

Provide feedback