Improve disposal of tensors during training (tensorflow#1604)

BUG
Improve memory management of tensors during training. Op authors now explicitly save intermediate tensors that are needed for backwards mode. This allows the engine to optimally dispose memory during forward pass, and keep only tensors needed for the backward pass.

Based on the [layers benchmark](https://github.com/tensorflow/tfjs-layers/tree/v1.0.0/integration_tests/benchmarks), this change along with tensorflow#1621 led to:

## 2-3X memory reduction
**Before**
![before-mem](https://user-images.githubusercontent.com/2294279/54299829-13b9db80-4592-11e9-97a4-04a95012b5c4.png)

**After**
![after-mem](https://user-images.githubusercontent.com/2294279/54299854-203e3400-4592-11e9-8887-62165d0bfbbb.png)


## 1.5-1.7x improvement in fit() for GRU and LSTM ops.

**Before** 
![before](https://user-images.githubusercontent.com/2294279/54299715-d5242100-4591-11e9-8a4c-b5944e991f57.png)

**After**
![after](https://user-images.githubusercontent.com/2294279/54299724-d9e8d500-4591-11e9-8ea9-e340e10a41ce.png)

 

- When the user writes the forward pass of an op, they are given a `save` function that allows them to save inputs or intermediate tensors to be reused for the backwards pass
- Before this change, the `save` function was a no-op, a placeholder for when we decide to optimize disposal of tensors in the training process in the future.
- However, `save` being a no-op caused a bug for existing users who rely on it (e.g. Magenta.js).
- After this change, the `save` function makes a shallow copy of the tensor, and keeps it until the backwards pass is done.
- `save` used to take an array of tensors. Now it takes a `NamedTensorMap` which improves code readability, and reduces chances of off-by-one index bugs.

Fixes tensorflow/tfjs#1320

PERF
BUG

Author: dsmilkov

karma.conf.js

@@ -19,7 +19,8 @@ const karmaTypescriptConfig = {
   tsconfig: 'tsconfig.json',
   // Disable coverage reports and instrumentation by default for tests
   coverageOptions: {instrumentation: false},
-  reports: {}
+  reports: {},
+  bundlerOptions: {sourceMap: true}
 };
 
 // Enable coverage reports and instrumentation under KARMA_COVERAGE=1 env
@@ -48,7 +49,7 @@ module.exports = function(config) {
     exclude: ['src/test_node.ts'],
     preprocessors: {'**/*.ts': ['karma-typescript']},
     karmaTypescriptConfig,
-    reporters: ['progress', 'karma-typescript'],
+    reporters: ['dots', 'karma-typescript'],
     browsers: ['Chrome'],
     browserStack: {
       username: process.env.BROWSERSTACK_USERNAME,