Tune function rangeI

[wangkuiyi]

The current definition is here

gotorch/example/resnet/resnet.go

Lines 31 to 40 in 9010c80

	func rangeI(n int64) []int64 {
	res := []int64{}
	if n <= 0 {
	return res
	}
	for i := int64(0); i < n; i++ {
	res = append(res, i)
	}
	return res
	}

I noticed some issues with this function.

1. The frequent call to append might be expensive. Each call might deallocate and reallocate and copy existing slice data. Instead, we can make([]int64, n), then copy each generated element to the right place.

2. This function would crash the program if the parameter n is too large. It would be safer to add a check to panic if n is larger than a threshold.