Fix gcc-9 build error: duplicate ‘const’ (#4016)
```
/home/byronyi/pytorch/xla/torch_xla/csrc/aten_autograd_ops.cpp: In static member function ‘static at::Tensor torch_xla::aten_autograd_ops::EinsumAutogradFunction::forward(torch::autograd::AutogradContext*, c10::string_view, at::TensorList)’:
/home/byronyi/pytorch/xla/torch_xla/csrc/aten_autograd_ops.cpp:28:28: error: duplicate ‘const’
28 | for (const torch::Tensor const& tensor : tensors) {
| ^~~~~
| -----
```
