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Update base for Update on "Distributed Autograd - FAST mode backward pass implementation."
 [test all] This change implements the "FAST" mode distributed autograd backward pass as described in #23110. At a high level the backward pass works as follows: 1. We start by computing dependencies on the node that calls `torch.distributed.backward`. 2. This node computes the dependencies starting from the root nodes provided in the backward call and all the 'send' functions present in the current autograd context. The "FAST" mode assumes all 'send' functions are part of the autograd computation. 3. Once the dependency computation is done, the distributed autograd engine calls the local autograd engine to execute the autograd graph. Note that the autograd graph on a single node is not necessarily connected because of inter-node communication. As a result, we have special handling to ensure the local autograd engine ensures we execute the entire graph starting from the provided roots and all 'send' functions on the node. 4. When the local autograd engine hits a 'recv' function, it performs an async RPC to send the gradients over to the appropriate node and stores a future in the autograd context to keep track of this RPC. 5. On the destination node, the appropriate 'send' function is looked up and enqueued on the local autograd engine. If this is the first time the node is hearing about this autograd context id on the backward pass, then the node computes dependencies for the local autograd engine. 6. As part of compute dependencies, the distributed autograd engine discovers all leaf nodes and ensures those are passed as 'outputs' to the local autograd engine. This avoids running the 'AccumulateGrad' function. 7. The gradients computed for the leaf nodes are then actually accumulated in `DistAutogradContext` for the appropriate autograd context id. 8. The distributed autograd engine waits for the local autograd engine to complete and also waits for all the 'Futures' (stored in 4.) for respective RPCs to finish. We have made the following changes to the local autograd engine for this purpose: 1. Expose GraphTask and NodeTask so that the distributed autograd engine can use them. 2. Expose a `execute_with_graph_task` API which gives the distributed engine to build a GraphTask and pass it to the local autograd engine. 3. Expose a `enqueue_on_cpu` API, which allows the distributed engine to build a `NodeTask` for a 'send' function and enqueue it on the local autograd engine. In addition to this a few general improvements: 1. Added a `PropagateGradients` RPC call for the 'recv' function to pass gradients to the appropriate node during the backward pass. 2. Use IValues as much as possible in serialization for RpcWithAutograd. 3. If Future.wait(), contains a message type EXCEPTION, we throw an appropriate exception instead of just returning the message. This is inline with what most Future.wait() APIs do. This was mostly done to ensure Future.wait() propagates errors c 7A37 orrectly on the backward pass. 4. Added a `get_gradients(context_id)` API which allows users to retrieve a map from Tensor to respective gradient for the provided context_id on the local node. Differential Revision: [D17652615](https://our.internmc.facebook.com/intern/diff/D17652615/) [ghstack-poisoned]
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