This PR adds a TestInjectionPolicy inference unittest class for testing custom injection policies.

This test differs from the existing tests in that the injection_policy dictionary is explicitly specified when calling the DeepSpeed init_inference API.

The google/t5-v1_1-small text2text-generation model and the roberta-large fill-mask model are added as tests with the injection policy explicitly specified.

This is done to expand our unittest coverage to test the path where the replace_wo_policy function is invoked (see GH-2387).
Co-authored-by: NLev Kurilenko <lekurile@microsoft.com>
Co-authored-by: NMichael Wyatt <michaelwyatt@microsoft.com>