Fix scalar multiplication

[bezineb5]

Hello,
I found an issue in the Mul operator, when the 1st term is a scalar, it always returned the 1st term unchanged.
The actual problem seems to be in defaultengine_arith.go, in MulScalar, where the leftTensor parameter is not properly handled (seems that on line 669, it calls Mul in a way that doesn't change the output). However, this is generated code, so I'm not sure how to fix it. Plus, it's actually simpler to commute the 2 terms.
What was the rationale behind the use of leftTensor?

Thanks!

[coveralls]

Coverage increased (+0.02%) to 72.809% when pulling f719761 on bezineb5:scalar-mul into d78b17f on gorgonia:master.

[coveralls]

Coverage increased (+0.003%) to 72.794% when pulling f719761 on bezineb5:scalar-mul into d78b17f on gorgonia:master.

[chewxy]

Two things

1. You Found A Bug

You have found a bug! Thank you! The bug is highly likely related to the issues you found, but not what you described.

The test cases you wrote highlights the bug:

	a2 := New(WithBacking([]float64{2}))
	b2 := New(WithBacking([]float64{3}))
	var correct interface{} = 6.0

	res, err := Mul(a2, b2)
	if err != nil {
		t.Fatalf("Error: %v", err)
	}
	assert.Equal(t, correct, res.Data())
	t.Logf("a2 %v b2 %v, res %v", a2, b2, res)

and

	a := New(WithBacking([]float64{3, 2}))
	b := New(WithBacking([]float64{2}))
	correct := []float64{6, 4}

	res, err := Mul(a, b)
	if err != nil {
		t.Fatalf("Error: %v", err)
	}
	assert.Equal(t, correct, res.Data())
	t.Logf("a %v b %v, res %v", a, b, res)

	// Test commutativity
	res, err = Mul(b, a)
	if err != nil {
		t.Fatalf("Error: %v", err)
	}
	assert.Equal(t, correct, res.Data())
	t.Logf("a %v b %v, res %v", a, b, res)

The solution however is not to fix the API, but to fix the defaultEngine 's arith functions, which are as you say, generated.

Specifically, one line short of 666: https://github.com/gorgonia/tensor/blob/master/defaultengine_arith.go#L665

This problem is easily fixed. But for now I'd like to close this PR, write up an issue, then attribute the fix to you

2. Email Me

Please email me - chewxy [at] gmail.com