flop count analysis of LSTM layers

Hello, it seems that LSTM layers are not yet supported for the `fvcore.nn.FlopCountAnalysis` method:

```python
import torch
from fvcore.nn import FlopCountAnalysis
from torch import nn

class ToyLSTMModel(nn.Module):
def __init__(self):
super().__init__()
self.rnn = nn.LSTM(10, 20, 1)

def forward(self, x):
h0 = torch.randn(1, 3, 20)
c0 = torch.randn(1, 3, 20)
output, _ = self.rnn(x, (h0, c0))

return output

model = ToyLSTMModel()
example_input = torch.randn(5, 3, 10)

print(FlopCountAnalysis(model, example_input).by_module())
```

gives:
```
Unsupported operator aten::randn encountered 2 time(s)
Unsupported operator aten::lstm encountered 1 time(s)
Counter({'': 0, 'rnn': 0})
```
While the same works an LSTM cell:

```python
import torch
from fvcore.nn import FlopCountAnalysis
from torch import nn

class ToyLSTMModel(nn.Module):
def __init__(self):
super().__init__()
self.rnn = nn.LSTMCell(10, 20)

def forward(self, x):
hx = torch.randn(3, 20)
cx = torch.randn(3, 20)
output = []
for i in range(x.size()[0]):
hx, cx = self.rnn(x[i], (hx, cx))
output.append(hx)
output = torch.stack(output, dim=0)
return output

model = ToyLSTMModel()
example_input = torch.randn(5, 3, 10)

print(FlopCountAnalysis(model, example_input).by_module())
```
output:
```
Unsupported operator aten::randn encountered 2 time(s)
Unsupported operator aten::add_ encountered 10 time(s)
Unsupported operator aten::unsafe_chunk encountered 5 time(s)
Unsupported operator aten::sigmoid_ encountered 15 time(s)
Unsupported operator aten::tanh_ encountered 5 time(s)
Unsupported operator aten::mul encountered 15 time(s)
Unsupported operator aten::tanh encountered 5 time(s)
Counter({'': 36000, 'rnn': 36000})
```
Is there any particular reason for that? The number of FLOPS of the LSTM layer should be the same than from one LSTM cell times the number of time steps.