CPU/GPU memory benchmarking utilities - Remove support for python 3.5 (now only 3.6+)
This PR add some utilities to benchmark (RAM) memory consumption of the models.
This is actually a generic utility that can work with any arbitrary python code

Ex:
```python
import torch
from transformers import GPT2Model, GPT2Tokenizer
from transformers import start_memory_tracing, stop_memory_tracing

tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
model = GPT2Model.from_pretrained('gpt2')

sequence = tokenizer.encode("Hello how are you", return_tensors='pt')

# Line by line memory tracing (all code in the module `transformers`).
trace = start_memory_tracing(modules_to_trace="transformers")
output = model(sequence)
summary = stop_memory_tracing(trace)

# Summary contain three fields:
# `sequential`: list of line by line consumption (with line code and location)
# `cumulative`: list of cumulative line by line consumption (when lines are executed several times) ordered from the most memory consuming line to the least (also with line code and location)
# `total`: total memory consumption of the script (default to sum memory increase at each line and ignore released mem, can be seet to count increase and release by less reliable on ubuntu).
# Each `Memory` object contain CPU, GPU and CPU + GPU memory, each both in int and human readable string

print(f"Total memory consumption: {summary.total}")
top_line = summary.cumulative[0]
print(f"Consumed {top_line.cpu_gpu}: {top_line.frame.line_text} at {top_line.frame.filename}:{top_line.frame.line_number}")
```

Incorporated in the `./examples/benchmark.py` script. Example of command-line run:
``` bash
(py37) bash-3.2$ python ./examples/benchmarks.py --models gpt2 --torch --batch_sizes 1 --slice_sizes 64 256 512 512 512 --no_speed --verbose
Running with arguments Namespace(amp=False, average_over=30, batch_sizes=[1], csv_filename=None, fp16=False, keras_predict=False, models=['gpt2'], no_memory=False, no_speed=True, save_to_csv=False, slice_sizes=[64, 256, 512, 512, 512], tensorflow=False, torch=True, torch_cuda=False, torchscript=False, verbose=False, xla=False)
1 / 1
Token indices sequence length is longer than the specified maximum sequence length for this model (2708 > 1024). Running this sequence through the model will result in indexing errors
....
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:487: mem 0.000B:                 presents = presents + (present,)
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:489: mem 0.000B:             if self.output_attentions:
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:477: mem 0.000B:         for i, (block, layer_past) in enumerate(zip(self.h, past)):
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:492: mem 0.000B:         hidden_states = self.ln_f(hidden_states)
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:494: mem 0.000B:         hidden_states = hidden_states.view(*output_shape)
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:496: mem 0.000B:         if self.output_hidden_states:
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:499: mem 0.000B:         outputs = (hidden_states,)
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:500: mem 0.000B:         if self.output_past:
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:501: mem 0.000B:             outputs = outputs + (presents,)
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:502: mem 0.000B:         if self.output_hidden_states:
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:504: mem 0.000B:         if self.output_attentions:
/Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:509: mem 0.000B:         return outputs  # last hidden state, (presents), (all hidden_states), (attentions)

Top 5 script lines consuming the most memory:
0 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/activations.py:31: mem 276.004MB:     return 0.5 * x * (1 + torch.tanh(math.sqrt(2 / math.pi) * (x + 0.044715 * torch.pow(x, 3))))
1 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_utils.py:1311: mem 151.520MB:         x = torch.addmm(self.bias, x.view(-1, x.size(-1)), self.weight)
2 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:146: mem 146.004MB:         w = w * b - 1e4 * (1 - b)
3 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:143: mem 132.004MB:             w = w / math.sqrt(v.size(-1))
4 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:187: mem 36.000MB:         present = torch.stack((key.transpose(-2, -1), value))  # transpose to have same shapes for stacking
5 => /Users/thomwolf/Documents/GitHub/transformers/src/transformers/modeling_gpt2.py:159: mem 33.000MB:         outputs = [torch.matmul(w, v)]

Memory increase computed by summing traced script lines: 843.758MB
=========== RESULTS ===========
        ======= MODEL CHECKPOINT: gpt2 =======
                ===== BATCH SIZE: 1 =====
                gpt2/1/64: N/A 75.176MB
                gpt2/1/256: N/A 349.695MB
                gpt2/1/512: N/A 843.758MB
                gpt2/1/512: N/A 843.758MB
                gpt2/1/512: N/A 843.758MB
```
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=h1) Report
> Merging [#3186](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e03129ad447ad7670fcc6206e5eb27a5435d4d86&el=desc) will **decrease** coverage by `0.50%`.
> The diff coverage is `30.76%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3186/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3186      +/-   ##
==========================================
- Coverage   78.15%   77.64%   -0.51%     
==========================================
  Files          98       99       +1     
  Lines       16641    16795     +154     
==========================================
+ Hits        13006    13041      +35     
- Misses       3635     3754     +119     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.34% <15.38%> (-3.07%)` | :arrow_down: |
| [src/transformers/benchmark\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9iZW5jaG1hcmtfdXRpbHMucHk=) | `31.74% <31.74%> (ø)` | |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.92% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/configuration\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2dwdDIucHk=) | `97.29% <100.00%> (+0.07%)` | :arrow_up: |
| [src/transformers/modeling\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19ncHQyLnB5) | `86.07% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3186/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `91.53% <0.00%> (-2.17%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=footer). Last update [e03129a...cb67ca6](https://codecov.io/gh/huggingface/transformers/pull/3186?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
