In the poem "Ode to Autumn" by John Keats, who is the speaker and to whom is he speaking?

The speaker in "Ode to Autumn" is unspecified; the poem never directly uses "I." Because we don't have any information about the speaker as a character, we can assume that the speaker is simply an unspecified, omniscient poetic narrator. 
We do know, however, that this speaker is somehow able to follow autumn around from place to place as it rests in granaries, beside brooks, and so forth. The speaker employs a kind of zooming-in feature, examining several of the small, rich details and images of autumn. The speaker zooms in on these details in order to help us readers truly notice these often overlooked moments in autumn, a time when many are too busy grieving the end of summer to notice the beauty of this unique season.
The speaker addresses the season of autumn in this poem, which we immediately know from the poem's title, "Ode to Autumn." Autumn, however, takes on a few possible forms here. We see it sitting "careless on a granary floor," with its hair "soft-lifted by the winnowing wind" in a slightly melancholy yet drowsy and peaceful image. Autumn could be either a man or a woman. "Sitting on a granary floor" suggests a male farmer, but later on "laden head" suggests a woman carrying a basket of fruit on her head. Keats leaves autumn's gender ambiguous and instead allows it to be an androgynous character. Autumn thus resembles a kind of god or goddess of harvest, fruition, and plenty—perhaps Demeter, Greek goddess of the harvest, or Dionysus, Greek god of winemaking and ritual.