Simplify tf.ENV.registerBackend (remove setTensorTracker param) (tensorflow#1517)

INTERNAL

There is no need for custom backends to provide their own tensor trackers since the tracker is the engine (a global).

Tested that double tfjs import works:
```html
<script src="http://github.com/tf-core.min.js"></script>
<script src="http://github.com/tf-core.min.js"></script>
<script>
	tf.square(3).print();
</script>
```

Author: dsmilkov

src/canvas_util.ts

@@ -29,11 +29,6 @@ const WEBGL_ATTRIBUTES: WebGLContextAttributes = {
 
 export function getWebGLContext(webGLVersion: number): WebGLRenderingContext {
   if (!(webGLVersion in contexts)) {
-    const canvas = document.createElement('canvas');
-    canvas.addEventListener('webglcontextlost', ev => {
-      ev.preventDefault();
-      delete contexts[webGLVersion];
-    }, false);
     contexts[webGLVersion] = getWebGLRenderingContext(webGLVersion);
   }
   const gl = contexts[webGLVersion];
@@ -61,6 +56,10 @@ function getWebGLRenderingContext(webGLVersion: number): WebGLRenderingContext {
   }
 
   const canvas = document.createElement('canvas');
+  canvas.addEventListener('webglcontextlost', ev => {
+    ev.preventDefault();
+    delete contexts[webGLVersion];
+  }, false);
   if (webGLVersion === 1) {
     return (canvas.getContext('webgl', WEBGL_ATTRIBUTES) ||
             canvas.getContext('experimental-webgl', WEBGL_ATTRIBUTES)) as

src/environment.ts

@@ -19,7 +19,7 @@ import * as device_util from './device_util';
 import {Engine, MemoryInfo, ProfileInfo, ScopeFn, TimingInfo} from './engine';
 import {Features, getFeaturesFromURL, getMaxTexturesInShader, getNumMBBeforePaging, getWebGLDisjointQueryTimerVersion, getWebGLMaxTextureSize, isChrome, isDownloadFloatTextureEnabled, isRenderToFloatTextureEnabled, isWebGLFenceEnabled, isWebGLVersionEnabled} from './environment_util';
 import {KernelBackend} from './kernels/backend';
-import {DataId, setTensorTracker, Tensor, TensorTracker} from './tensor';
+import {DataId, setTensorTracker, Tensor} from './tensor';
 import {TensorContainer} from './tensor_types';
 import {getTensorsInContainer} from './tensor_util';
 
@@ -416,15 +416,11 @@ export class Environment {
    *     the best backend. Defaults to 1.
    * @return False if the creation/registration failed. True otherwise.
    */
-  registerBackend(
-      name: string, factory: () => KernelBackend, priority = 1,
-      setTensorTrackerFn?: (f: () => TensorTracker) => void): boolean {
+  registerBackend(name: string, factory: () => KernelBackend, priority = 1):
+      boolean {
     if (name in this.registry) {
       console.warn(
           `${name} backend was already registered. Reusing existing backend`);
-      if (setTensorTrackerFn != null) {
-        setTensorTrackerFn(() => this.engine);
-      }
       return false;
     }
     try {
@@ -480,8 +476,10 @@ function getOrMakeEnvironment(): Environment {
   const ns = getGlobalNamespace();
   if (ns.ENV == null) {
     ns.ENV = new Environment(getFeaturesFromURL());
-    setTensorTracker(() => ns.ENV.engine);
   }
+  // Tell the current tensor interface that the global engine is responsible for
+  // tracking.
+  setTensorTracker(() => ns.ENV.engine);
   return ns.ENV;
 }
 

src/kernels/backend_cpu.ts

@@ -32,7 +32,7 @@ import {buffer, scalar, tensor, tensor3d, tensor4d} from '../ops/ops';
 import * as scatter_nd_util from '../ops/scatter_nd_util';
 import * as selu_util from '../ops/selu_util';
 import {computeFlatOffset, getStridedSlicedInfo, isSliceContinous} from '../ops/slice_util';
-import {DataId, Scalar, setTensorTracker, Tensor, Tensor1D, Tensor2D, Tensor3D, Tensor4D, Tensor5D, TensorBuffer} from '../tensor';
+import {DataId, Scalar, Tensor, Tensor1D, Tensor2D, Tensor3D, Tensor4D, Tensor5D, TensorBuffer} from '../tensor';
 import {DataType, DataTypeMap, DataValues, NumericDataType, Rank, ShapeMap, TypedArray, upcastType} from '../types';
 import * as util from '../util';
 import {now} from '../util';
@@ -3363,5 +3363,4 @@ export class MathBackendCPU implements KernelBackend {
   }
 }
 
-ENV.registerBackend(
-    'cpu', () => new MathBackendCPU(), 1 /* priority */, setTensorTracker);
+ENV.registerBackend('cpu', () => new MathBackendCPU(), 1 /* priority */);

src/kernels/backend_webgl.ts

@@ -32,7 +32,7 @@ import * as segment_util from '../ops/segment_util';
 import {computeFlatOffset, getStridedSlicedInfo, isSliceContinous} from '../ops/slice_util';
 import {softmax} from '../ops/softmax';
 import {range, scalar, tensor} from '../ops/tensor_ops';
-import {DataId, Scalar, setTensorTracker, Tensor, Tensor1D, Tensor2D, Tensor3D, Tensor4D, Tensor5D} from '../tensor';
+import {DataId, Scalar, Tensor, Tensor1D, Tensor2D, Tensor3D, Tensor4D, Tensor5D} from '../tensor';
 import {DataType, DataTypeMap, DataValues, NumericDataType, Rank, RecursiveArray, ShapeMap, sumOutType, TypedArray, upcastType} from '../types';
 import * as util from '../util';
 import {getTypedArrayFromDType, sizeFromShape} from '../util';
@@ -2304,9 +2304,7 @@ export class MathBackendWebGL implements KernelBackend {
 }
 
 if (ENV.get('IS_BROWSER')) {
-  ENV.registerBackend(
-      'webgl', () => new MathBackendWebGL(), 2 /* priority */,
-      setTensorTracker);
+  ENV.registerBackend('webgl', () => new MathBackendWebGL(), 2 /* priority */);
 }
 
 function float32ToTypedArray<D extends NumericDataType>(