Hashmap was used twice by both Eben and I, also added error display w…

…hen client can't be start

src/core/graph/context.rs

@@ -1,5 +1,3 @@
-use std::collections::HashMap;
-
 use self::operation::{
     ConstantBinding, Node, NodeIdentifier, Operation, Parameter, ParameterBinding,
 };

src/core/graph/mod.rs

@@ -39,7 +39,7 @@ pub fn example() {
     let maybe_client = xla::PjRtClient::gpu(0.7, false);
     let client = match maybe_client {
         Ok(c) => c,
-        Err(_) => panic!("Failed to construct XLA client!")
+        Err(e) => panic!("Failed to construct XLA client!\n{}", e)
     };
     let name = "test";
     let executable = ctx.compile(sum, &name, &client);