Fix retries during inference

Author: borzunov

src/petals/client/inference_session.py

@@ -52,7 +52,7 @@ def __init__(
         self.stepped = False
         self.closed = False
 
-        self._position = 0
+        self.position = 0
         self.history = None  # Used in case of server failures to regenerate attention caches on new servers
         self.next_session = None
 
@@ -102,12 +102,11 @@ def step(
         n_input_tokens = inputs.shape[1]
         if self.history is None:
             self.history = inputs
-        elif self.history.shape[1] == self._position:
+        elif self.history.shape[1] == self.position:
             self.history = torch.cat([self.history, inputs[:, -n_input_tokens:]], dim=1)
-        assert self.history.shape[1] == self._position + n_input_tokens, (
-            f"Broken input cache: span={self.span} shape={self.history.shape} "
-            f"position={self._position} n_input_tokens={n_input_tokens}"
-        )
+        assert (
+            self.history.shape[1] == self.position + n_input_tokens
+        ), f"Broken input cache: {self.span=} {self.history.shape=} {self.position=} {n_input_tokens=}"
 
         if not self.stepped:
             inputs = self.history  # Pass full inputs including prefix
@@ -169,7 +168,7 @@ def step(
             outputs[0].shape == inputs.shape
         ), f"output activation shape is different from input shape: {outputs[0].shape} != {inputs.shape}"
 
-        self._position += n_input_tokens
+        self.position += n_input_tokens
 
         return outputs[0]
 
@@ -359,6 +358,10 @@ def _update_sequence(self, server_idx: int, block_idx: int, attempt_no: int) ->
         # If there is a failed span, this code replaces it, otherwise it just adds new ones
         if server_idx < n_prev_spans:
             updated_sessions[0].history = self._server_sessions[server_idx].history
+            updated_sessions[0].position = self._position
+            assert (
+                updated_sessions[0].history.shape[1] == self._position
+            ), f"Broken input cache: {updated_sessions[0].history.shape[1]=} {self._position=}"
         self._server_sessions[server_idx : server_idx + 1] = updated_sessions
 
         # Update links to the next server session for direct server-to-server communication via rpc_push()