Move mnist crate into dev-dependencies in cargo manifest

Author: 9prady9

Cargo.toml

@@ -53,13 +53,13 @@ libc = "0.2"
 num  = "0.4.0"
 lazy_static = "1.0"
 half = "1.5.0"
-mnist = {version="0.5.0", features = ["download"]}
 serde = { version = "1.0", features = ["derive"], optional = true }
 
 [dev-dependencies]
 half = "1.5.0"
 serde_json = "1.0"
 bincode = "1.3"
+mnist = {version="0.5.0", features = ["download"]}
 
 [build-dependencies]
 serde_json = "1.0"