Add py.typed for type hints #1672

Job	Run time
Compatibility Checks (3.9)	2m 59s
Notebook Checks (ARENA_Content)	1m 5s
Code Checks	6m 11s
Compatibility Checks (3.10)	3m 0s
Notebook Checks (Colab_Compatibility)	1m 17s
Notebook Checks (BERT)	1m 21s
Notebook Checks (Exploratory_Analysis_Demo)	5m 32s
Notebook Checks (Interactive_Neuroscope)	1m 31s
Notebook Checks (Main_Demo)	2m 0s
Notebook Checks (Othello_GPT)	1m 6s
Notebook Checks (Patchscopes_Generation_Demo)	6m 20s
Build Docs	0s
Deploy Docs	0s
	32m 22s

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