Revert "[AutoDiff] Forward-mode support for variables, generics, tuples, structs."

[dan-zheng]

Reverts #26743.

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This PR likely causes swift test for tensorflow/swift-apis to fail verification:

SIL verification failed: switch_enum dispatches on same enum element more than once: unswitchedElts.count(elt)
Verifying instruction:
     %2 = struct_extract %1 : $_AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__PB__src_0_wrt_3<τ_0_0, τ_1_0>, #_AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__PB__src_0_wrt_3.predecessor // user: %91
->   switch_enum %2 : $_AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__Pred__src_0_wrt_3<τ_0_0, τ_1_0>, case #_AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__Pred__src_0_wrt_3.bb5!enumelt.1: bb8, case #_AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__Pred__src_0_wrt_3.bb4!enumelt.1: bb1 // id: %91
In function:
// AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF__pullback_src_0_wrt_3
sil hidden [serialized] [canonical] @AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF__pullback_src_0_wrt_3 : $@convention(method) <τ_0_0 where τ_0_0 : TensorFlowFloatingPoint><τ_1_0 where τ_1_0 : TensorFlowIndex> (@guaranteed Tensor<τ_0_0>, @owned _AD__$s10TensorFlow0A0V14batchGathering9atIndices9alongAxis0C14DimensionCountACyxGACyqd__G_S2itAA0aB5IndexRd__lF_bb6__PB__src_0_wrt_3<τ_0_0, τ_1_0>) -> @owned Tensor<τ_0_0>

Full verification failure log.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

Closing, as this PR is not the cause of the verification failure.

The failure is related to IterableDeclContext::loadAllMembers for AD-generated structs and enums. Related commits are:

• Upstream patch: [Parser] Introduce a request for parsing type and extension members..
• tensorflow merge patch: 7e89bee.
  • Reverting this patch makes the verification failure go away, but causes many other test failures.