Fix several issues with `go fmt` and `go lint`

This fix fixes several issues related to `go fmt` and `go lint`
based on https://goreportcard.com/report/github.com/tensorflow/tensorflow



There are several changes:
- `gofmt -s tensorflow/go/tensor.go`
- `gofmt -s tensorflow/go/example_inception_inference_test.go`
- `golint tensorflow/go/genop/internal/lib.go`

At the moment there are still quite a few golint and ineffassign warnings
in the current go code base. However, all of them are from `tensorflow/go/op/wrappers.go`
which is machine generated code.

This fix does not cover `tensorflow/go/op/wrappers.go`.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>