Pass Scalar by reference (#2817)
* Treat Scalar parameter as if it is constant
We will pass `Scalar` by reference in the following commit,
i.e. `const Scalar&`.
* Pass Scalar by reference
Commands used:
```
fastmod '([a-zA-Z_+]\([^)]*,?\s*)at::Scalar (\w+)' '${1}const at::Scalar& ${2}'
fastmod '([a-zA-Z_+]\([^)]*,?\s*)c10::optional<at::Scalar> (\w+)' '${1}const c10::optional<at::Scalar>& ${2}'
```
Then run clang-format-7:
```
clang-format-7 -i -style=file \
torch_xla/csrc/aten_xla_type.cpp \
torch_xla/csrc/aten_xla_type.h \
torch_xla/csrc/elementwise.cpp \
torch_xla/csrc/elementwise.h \
torch_xla/csrc/ops/hardtanh_backward.cpp \
torch_xla/csrc/ops/masked_fill.cpp \
torch_xla/csrc/ops/ops.cpp \
torch_xla/csrc/ops/ops.h \
torch_xla/csrc/ops/rrelu_with_noise_backward.cpp \
torch_xla/csrc/ops/rrelu_with_noise_backward.h \
torch_xla/csrc/tensor.h \
torch_xla/csrc/tensor_methods.cpp
```
