Fix type of `get_all` gRPC call

[rafilong]

The type for the generated batch_get_all is AsyncIterable[firestore.BatchGetDocumentsResponse], but was incorrectly mocked as Iterable[firestore.BatchGetDocumentsResponse].

[crwilcox]

@rafilong can you open this against https://github.com/googleapis/gapic-generator-python as well?

[rafilong]

Sorry, I just realized my description was a bit misleading. The incorrectly mocked type was in the Firestore test I wrote to test get_all, in which I was mocking the return type of batch_get_all. The generated code functioned fine, my use of it was slightly flawed.