lit shader with normal map

lit_normal.shader

Shader "MyShader/SimpleLit"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _NormalTex ("Normal", 2D) = "white" {}
        _Light ("Light", Vector) = (1,1,1)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                float3 normal : NORMAL;
                float4 tangent : TANGENT;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float3 light_dir : TEXCOORD2;
            };

            sampler2D _MainTex;
            sampler2D _NormalTex;
            float4 _MainTex_ST;
            float3 _Light;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                float3 light = UnityWorldToObjectDir(_Light);
                TANGENT_SPACE_ROTATION;
                o.light_dir = mul(rotation, light);
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = tex2D(_MainTex, i.uv);

                float3 normal = UnpackNormal(tex2D(_NormalTex, i.uv));
                col *= saturate(dot(normal, i.light_dir));
                return col;
            }
            ENDCG
        }
    }
}