"unreachable code" panic on certain uses of `grad_with_default`

[elidupree]

I ran into this panic during actual usage, then reduced it to these tests (borrowing test code from test_tensor_ops_grad.rs). It seems like maybe it's trying to take the gradient of a variable or placeholder when it shouldn't have to because grad_with_default provides the gradient explicitly...

#[test]
fn grad_with_default_variable() {
    let mut ctx = ag::VariableEnvironment::new();
    let v1 = ctx.slot().set(ndarray::arr1(&[1., 1., 1.]));
    ctx.run(|graph| {
        let v1 = graph.variable(v1);
        let g = T::grad_with_default(&[v1], &[v1], &[v1]);
        ag::test_helper::check_theoretical_grads(
            v1,
            g.as_slice(),
            &[v1],
            ag::Feeder::new(),
            1e-3,
            1e-3,
            graph,
        );
    });
}

#[test]
fn grad_with_default_placeholder() {
    ag::run(|ctx| {
        let mut eval = ctx.evaluator();
        let v1 = ctx.placeholder("v1", &[3]);
        let v1_val = ndarray::arr1(&[1., 1., 1.]);
        eval.feed("v1", v1_val.view());
        let g = T::grad_with_default(&[v1], &[v1], &[v1]);
        eval.extend(&g).run();
    });
}

---- test_tensor_ops_grad::add_n_single_placeholder stdout ----
thread 'test_tensor_ops_grad::add_n_single_placeholder' panicked at 'internal error: entered unreachable code', /n/rust-autograd/src/tensor_ops/basic_source_ops.rs:23:1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/panicking.rs:48:5
   3: <autograd::tensor_ops::basic_source_ops::Placeholder as autograd::op::Op<T>>::grad
             at ./src/tensor_ops/basic_source_ops.rs:15:17
   4: autograd::op::GradientContext<T>::compute_input_grads
             at ./src/op.rs:371:9
   5: autograd::gradient::compute_gradients
             at ./src/gradient.rs:60:23
   6: autograd::tensor_ops::grad_with_default
             at ./src/tensor_ops/mod.rs:142:21
   7: lib::test_tensor_ops_grad::add_n_single_placeholder::{{closure}}
             at ./tests/test_tensor_ops_grad.rs:83:17
   8: autograd::graph::run
             at ./src/graph.rs:99:5
   9: lib::test_tensor_ops_grad::add_n_single_placeholder
             at ./tests/test_tensor_ops_grad.rs:78:5
  10: lib::test_tensor_ops_grad::add_n_single_placeholder::{{closure}}
             at ./tests/test_tensor_ops_grad.rs:77:1
  11: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

---- test_tensor_ops_grad::add_n_single_variable stdout ----
thread 'test_tensor_ops_grad::add_n_single_variable' panicked at 'internal error: entered unreachable code', /n/rust-autograd/src/tensor_ops/basic_source_ops.rs:21:1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/panicking.rs:48:5
   3: <autograd::tensor_ops::basic_source_ops::Variable as autograd::op::Op<T>>::grad
             at ./src/tensor_ops/basic_source_ops.rs:15:17
   4: autograd::op::GradientContext<T>::compute_input_grads
             at ./src/op.rs:371:9
   5: autograd::gradient::compute_gradients
             at ./src/gradient.rs:60:23
   6: autograd::tensor_ops::grad_with_default
             at ./src/tensor_ops/mod.rs:142:21
   7: lib::test_tensor_ops_grad::add_n_single_variable::{{closure}}
             at ./tests/test_tensor_ops_grad.rs:63:17
   8: autograd::variable::VariableEnvironment<F>::run
             at ./src/variable.rs:697:9
   9: lib::test_tensor_ops_grad::add_n_single_variable
             at ./tests/test_tensor_ops_grad.rs:61:5
  10: lib::test_tensor_ops_grad::add_n_single_variable::{{closure}}
             at ./tests/test_tensor_ops_grad.rs:58:1
  11: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/897e37553bba8b42751c67658967889d11ecd120/library/core/src/ops/function.rs:248:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.