[AutoDiff] Enable differentiation wrt "subset indices". (#23887)

Remove this error: `function is differentiable only with respect to a
smaller subset of arguments`.

The differentiation pass (and "minimal indices" logic) actually supports
such differentiation wrt "subset indices". All that is needed is to remove
the deprecated error.

Author: dan-zheng

include/swift/AST/DiagnosticsSIL.def

@@ -396,9 +396,6 @@ NOTE(autodiff_protocol_member_not_differentiable,none,
 NOTE(autodiff_protocol_member_subset_indices_not_differentiable,none,
      "member is differentiable only with respect to a smaller subset of "
      "arguments", ())
-NOTE(autodiff_function_subset_indices_not_differentiable,none,
-     "function is differentiable only with respect to a smaller subset of "
-     "arguments", ())
 NOTE(autodiff_function_assoc_func_requirements_unmet,none,
      "function call is not differentiable because generic requirements are not "
      "met", ())

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -1793,13 +1793,6 @@ emitAssociatedFunctionReference(ADContext &context, SILBuilder &builder,
     if (autodiffFnType->isDifferentiable()) {
       SILValue assocFn = builder.createAutoDiffFunctionExtract(
           original.getLoc(), kind, /*differentiationOrder*/ 1, functionSource);
-      if (autodiffFnType->getDifferentiationParameterIndices().test(
-              desiredIndices.parameters)) {
-        context.emitNondifferentiabilityError(
-            original, parentTask,
-            diag::autodiff_function_subset_indices_not_differentiable);
-        return None;
-      }
       SILAutoDiffIndices indices(0, desiredIndices.parameters);
       return std::make_pair(assocFn, indices);
     }

test/AutoDiff/simple_math.swift

@@ -308,4 +308,12 @@ SimpleMathTests.test("StructGeneric") {
   expectEqual(405, gradient(at: 3, in: fifthPower))
 }
 
+SimpleMathTests.test("SubsetIndices") {
+  func train(_ lossFunction: @differentiable (Float, Float) -> Float) {
+    let y = Float(0)
+    _ = gradient(at: 0) { x in lossFunction(x, y) }
+  }
+  train { x, y in x + y }
+}
+
 runAllTests()