Merge pull request #2654 from dzhwinter/client_gradient

"fix client send empty gradients bug"

go/pserver/client.go

 package pserver
 
 import (
+	"errors"
 	"hash/fnv"
 	"sort"
 	"time"
@@ -123,6 +124,9 @@ func (c *Client) FinishInitParams() error {
 // SendGrads sends gradients to parameter servers for updating
 // parameters.
 func (c *Client) SendGrads(grads []Gradient) error {
+	if len(grads) == 0 {
+		return errors.New("no gradient received")
+	}
 	errCh := make(chan error, len(grads))
 	for _, g := range grads {
 		go func(g Gradient) {