"fix client send empty gradients bug"

[dzhwinter]

In some miss use situation, SendGrads will get empty slice here, then errCh will block forever.

// SendGrads sends gradients to parameter servers for updating
// parameters.
func (c *Client) SendGrads(grads []Gradient) error {
  if len(grads) == 0 {
    log.Info("Send Empty Gradient")
    return nil
  }
  errCh := make(chan error, len(grads))
  for _, g := range grads {
    go func(g Gradient) {
      err := c.pservers[c.partition(g.Name)].Call("Service.SendGrad", g, nil)
      errCh <- err
    }(g)
  }

  recv := 0
  for err := range errCh {
    if err != nil {
      return err
    }

[helinwang]

Maybe return an error. If we treat it as error, user can be aware of this error and fix it.

Btw, let's not use "Title Case" (基本每个单词都大写） for log message. For log message we can use a normal sentence like: "Sent empty gradient." (or people uses "sent empty gradient" (首单词不大写，不用句号结尾）)。

For error message, people typically use "sent empty gradient" (首单词不大写，不用句号结尾）, because error messages are often combined somewhere else. E.g.,

func foo() error {
  return errors.New("sent empty gradient")
}

func main() {
  err := foo()
  if err != nil {
    log.Errorf("foo returned error: %v", err)
  }
}

// Output:
// foo returned error: sent empty gradient

If we use a normal sentence like: "Sent empty gradient.", the output would be: "foo returned error: Sent empty gradient.", Which could be strange, because in English grammar, word typically does not capitalize after ":".

Also, "send empty gradient" may not express this error fully, "no gradient received" could be better.

[dzhwinter]

thanks for such a detailed comment! 👍👍👍
fix Done.

[helinwang]

Thank for noticing this! Good catch!

[helinwang]

Thanks! LGTM after fixing the only comment.