[Fit API] update estimator (apache#14849)

* address comments

* add comment

* check available context

* fix bug

* change cpu check

python/mxnet/gluon/contrib/estimator/estimator.py

@@ -88,24 +88,36 @@ def _check_metrics(self, metrics):
         return metrics
 
     def _check_context(self, context):
-        # handle context
-        if isinstance(context, Context):
-            context = [context]
-        elif isinstance(context, list) and all([isinstance(c, Context) for c in context]):
-            context = context
-        elif not context:
-            if num_gpus() > 0:
+        # infer available context
+        gpus = num_gpus()
+        available_gpus = [gpu(i) for i in range(gpus)]
+
+        if context:
+            # check context values, only accept Context or a list of Context
+            if isinstance(context, Context):
+                context = [context]
+            elif isinstance(context, list) and all([isinstance(c, Context) for c in context]):
+                context = context
+            else:
+                raise ValueError("context must be a Context or a list of Context, "
+                                 "for example mx.cpu() or [mx.gpu(0), mx.gpu(1)], "
+                                 "refer to mxnet.Context:{}".format(context))
+            for ctx in context:
+                assert ctx in available_gpus or str(ctx).startswith('cpu'), \
+                    "%s is not available, please make sure " \
+                    "your context is in one of: mx.cpu(), %s" % \
+                    (ctx, ", ".join([str(ctx) for ctx in available_gpus]))
+        else:
+            # provide default context
+            if gpus > 0:
                 # only use 1 GPU by default
-                if num_gpus() > 1:
+                if gpus > 1:
                     warnings.warn("You have multiple GPUs, gpu(0) will be used by default."
                                   "To utilize all your GPUs, specify context as a list of gpus, "
                                   "e.g. context=[mx.gpu(0), mx.gpu(1)] ")
                 context = [gpu(0)]
             else:
                 context = [cpu()]
-        else:
-            raise ValueError("context must be a Context or a list of Context, "
-                             "refer to mxnet.Context:{}".format(context))
         return context
 
     def _initialize(self, initializer):
@@ -167,7 +179,8 @@ def prepare_loss_and_metrics(self):
                 self.train_metrics = [Accuracy()]
             self.val_metrics = []
             for loss in self.loss:
-                self.train_metrics.append(Loss(''.join([i for i in loss.name if not i.isdigit()])))
+                # remove trailing numbers from loss name to avoid confusion
+                self.train_metrics.append(Loss(loss.name.rstrip('1234567890')))
             for metric in self.train_metrics:
                 val_metric = copy.deepcopy(metric)
                 metric.name = "Train " + metric.name

tests/python/unittest/test_gluon_estimator.py

@@ -258,6 +258,12 @@ def test_context():
                         metrics=metrics,
                         context='cpu')
 
+    with assert_raises(AssertionError):
+        est = Estimator(net=net,
+                        loss=loss,
+                        metrics=metrics,
+                        context=[mx.gpu(0), mx.gpu(100)])
+
 
 def test_categorize_handlers():
     class CustomHandler1(TrainBegin):