feat: add llm_usage to the exec_* config

src/pdl/pdl.py

@@ -58,6 +58,8 @@ class InterpreterConfig(TypedDict, total=False):
     """
     event_loop: AbstractEventLoop
     """Event loop to schedule LLM calls."""
+    llm_usage: PdlUsage
+    """Data structure where to accumulate LLMs usage."""
 
 
 class Result(TypedDict):

src/pdl/pdl_interpreter_state.py

@@ -36,12 +36,14 @@ class InterpreterState(BaseModel):
     """Cache containing the imported files."""
     event_loop: AbstractEventLoop = Field(default_factory=create_event_loop_thread)
     """Event loop to schedule LLM calls."""
-    current_pdl_context: Ref[LazyMessages] = Ref(DependentContext([]))
+    current_pdl_context: Ref[LazyMessages] = Field(
+        default_factory=lambda: Ref(DependentContext([]))
+    )
     """Current value of the context set at the beginning of the execution of the block."""
     replay: dict[str, Any] = {}
     """Dictionary that associate runtime block ids with their values to be able to replay an execution."""
     llm_usage: PdlUsage = PdlUsage()
-    """Record statistics about LLM usage."""
+    """Data structure where to accumulate LLMs usage."""
 
     def with_yield_result(self: "InterpreterState", b: bool) -> "InterpreterState":
         return self.model_copy(update={"yield_result": b})